Im getting the issue while passing my dropdown fields into to the controller

This is my Blade code for handling the form like my requirement is need to get the guard from the guard table need to store data in the rounds table as well as i need to store selected checkpoints
from the dropdown in the db table ,the guard data is storing perfectly but the checkpoints data is not storing im getting the error like this rounds rounds
3 / 4 requests
3.8 kB / 3.8 kB transferred
{message: “The selected checkpoints must be an array.”,…}
errors
:
{selected_checkpoints: [“The selected checkpoints must be an array.”]}
message
:
“The selected checkpoints must be an array.”

{!! Form::open(['class'=>'form-horizontal ajax_form','method'=>'POST','files' => true, 'autocomplete' => 'off']) !!}
<div class="row">
    <!-- Left Column -->
        <div class="col-md-6 col-sm-6">
            <div class="portlet light bordered">
                   <!-- Title Start  -->
                    <div class="portlet-title">
                        <div class="caption font-purple-wisteria">
                            <i class="fa fa-user font-purple-wisteria"></i>{!! trans('Round Details') !!}
                        </div>
                    </div>
                    <!-- Title  -->

                    <!-- Body Start -->
                    <div class="portlet-body">
                        <!-- Add your left column form elements here -->
                            <div class="form-group">
                                <label class="col-md-3 control-label">{{ trans('Select Guard')}} <span class="required">* </span></label>
                                <div class="col-md-9">
                                    <select class="form-control" name="guard">
                                        @foreach ($guard as $guard)
                                            <option value="{{ $guard->id }}">{{ $guard->full_name }}</option>
                                        @endforeach
                                    </select>
                                </div>
                            </div>

                            <div class="form-group">
                                <label class="col-md-3 control-label">{{ trans('RoundName')}}<span class="required">* </span></label>
                                <div class="col-md-9">
                                    <input type="text" class="form-control" name="roundname" id="roundname" placeholder="{{ trans('Roundname')}}" value="{{old('roundname')}}" disabled>
                                </div>
                            </div>

                            <div class="form-group">
                                <label class="col-md-3 control-label">{{ trans('StartTime')}}<span class="required">* </span></label>
                                <div class="col-md-9">
                                    <input type="time" class="form-control" name="shiftstartTime" id="shiftstartTime" placeholder="{{ trans('core.shiftstartTime')}}" value="{{old('shiftstartTime')}}" onchange="updateRoundName()">
                                </div>
                            </div>

                            <div class="form-group">
                                <label class="col-md-3 control-label">{{ trans('EndTime')}}<span class="required">* </span></label>
                                <div class="col-md-9">
                                    <input type="time" class="form-control" name="shiftendTime" id="shiftendTime" placeholder="{{ trans('core.shiftendTime')}}" value="{{old('shiftendTime')}}" onchange="updateRoundName()">
                                </div>
                            </div>

                            <hr>
                            <button type="button" style="margin: 1% 0 1% 25%;" onclick="addEmployee();return false;" class=" btn green ">
                                {{ trans('core.btnSubmit')}}
                            </button>
                
                            <!-- Add more left column form elements as needed -->
                        
                    </div>
                    <!-- Body End -->
            </div>
        </div>

    
    <!-- Right Column -->
        <div class="col-md-6 col-sm-6">
                <div class="portlet light bordered">
                    <!-- Title Start -->
                    <div class="portlet-title">
                        <!-- Add your right column form elements here -->
                        <div class="caption font-purple-wisteria">
                            <i class="fa fa-user font-purple-wisteria"></i>{!! trans('Add Your checkPoint Here!') !!}
                        </div>
                    </div>
                    <!-- Title End -->

                    <!-- Body Start -->

                    <div class="portlet-body">
                        <!-- Add your right column form elements here -->
                        <div class="form-group">
                            <div class="col-md-10">
                              <label>{{ trans('Select Checkpoint')}} <span class="required">* </span></label>
                              <input type="hidden" name="selected_checkpoints[]" id="selected_checkpoints" value="">
                              <select class="form-control" name="selected_checkpoints[]" id="checkpoint-select">
                                @foreach ($checkpoint as $checkpoint)
                                    <option value="{{ $checkpoint->id }}">{{ $checkpoint->checkpointname }} {{ $checkpoint->id }}</option>
                                @endforeach
                            </select>


                                <button type="button" class="btn btn-success btn-sm" style="margin: 4% 14% 0% 0%;" id="add-checkpoint" onclick="addCheckpoint()">Add</button>
                            </div>
                        </div>

                        <div class="form-group">
                            <div class="col-md-12">
                                <table id="selected-checkpoints-table" class="table" style="background-color:#eff1f4;">
                                    <thead>
                                        <tr>
                                            <th>Sr.No.</th>
                                            <th>Checkpoint Name</th>
                                            <th>Delete</th>
                                        </tr>
                                    </thead>
                                    <tbody>
                                        <!-- Selected checkpoints will be displayed here -->
                                    </tbody>
                                </table>
                            </div>
                        </div>
                        
                    </div>
                    <!-- Body End -->

                </div>
        </div>
        <!-- Right column End -->
</div>
{!! Form::close() !!}

My js code

<script>
function addEmployee() {
    // Clear any previous error messages
    $('.form-group').removeClass('has-error');
    $('.help-block').remove();

    var isValid = true;

    // Validate Full Name
    var roundName = $('input[name="roundname"]').val().trim();
    if (roundName === '') {
        isValid = false;
        $('input[name="roundname"]').closest('.form-group').addClass('has-error');
        $('input[name="roundname"]').after('<span class="help-block">Round name is required.</span>');
    }

    if (isValid) {
        // Collect selected checkpoint IDs
        var selectedCheckpointIds = Array.from(document.querySelectorAll("#selected-checkpoints-table tbody tr")).map(function (row) {
            return row.dataset.checkpointId;
        });

        // Attach selectedCheckpointIds to the form data
        $('input[name="selected_checkpoints"]').remove(); // Remove any previous hidden input
        $('<input>').attr({
            type: 'hidden',
            name: 'selected_checkpoints',
            value: JSON.stringify(selectedCheckpointIds)
        }).appendTo('.ajax_form');

        // If all validations pass, proceed with form submission
        var url = "{{ route('admin.rounds.store') }}";
        $.easyAjax({
            type: 'POST',
            url: url,
            container: '.ajax_form',
            file: true,
        });
    }
}


// updating the roundname with the start time and endtime..
    function updateRoundName() {
        var startTime = document.getElementById('shiftstartTime').value.replace(':', ''); //it Remove colons
        var endTime = document.getElementById('shiftendTime').value.replace(':', ''); // Remove colons
        var roundnameField = document.getElementById('roundname'); // getting the roundname using the id..

        // Combine the start time and end time without colons
        var roundname = startTime + '-' + endTime;

        // Update the "roundname" field value
        roundnameField.value = roundname;
    }



// Checkpoint data
// Initialize an array to store selected checkpoint IDs
var selectedCheckpointIds = [];
var rowCount = 1; // Initialize rowCount to 1

// Function to add the selected checkpoint to the selected_checkpoints field
function addSelectedCheckpoint(checkpointId) {
    var selectedCheckpointsInput = document.getElementById("selected_checkpoints");
    var selectedCheckpoints = selectedCheckpointsInput.value ? selectedCheckpointsInput.value.split(",") : [];

    // Add the checkpointId to the selectedCheckpoints array
    selectedCheckpoints.push(checkpointId);

    // Update the selected_checkpoints field with the updated array
    selectedCheckpointsInput.value = selectedCheckpoints.join(",");
}

// Function to remove the selected checkpoint from the selectedCheckpointIds array
function removeSelectedCheckpoint(checkpointId) {
    var index = selectedCheckpointIds.indexOf(checkpointId);
    if (index !== -1) {
        selectedCheckpointIds.splice(index, 1);
    }
}

function addCheckpoint() {
    var selectBox = document.getElementById("checkpoint-select");
    var selectedValue = selectBox.options[selectBox.selectedIndex].value;
    var selectedText = selectBox.options[selectBox.selectedIndex].text;

    if (selectedValue !== "") {
        // Check if the checkpoint has already been added
        var tableBody = document.querySelector("#selected-checkpoints-table tbody");
        var rows = tableBody.rows;
        for (var i = 0; i < rows.length; i++) {
            var existingValue = rows[i].cells[1].textContent;
            if (existingValue === selectedText) {
                alert("Checkpoint already added.");
                return;
            }
        }

        // Create a new row in the table and append it
        var newRow = tableBody.insertRow(tableBody.rows.length);
        newRow.insertCell(0).textContent = rowCount++; // Increment rowCount
        newRow.insertCell(1).textContent = selectedText;
        var deleteCell = newRow.insertCell(2);
        var deleteButton = document.createElement("button");
        deleteButton.textContent = "Delete";
        deleteButton.className = "btn btn-danger";
        deleteButton.onclick = function () {
            deleteRow(newRow);
            addOptionToSelect(selectedValue, selectedText);
            updateSno(); // Update the Sr.No.
            updateDropdownMessage(); // Update the dropdown message
            removeSelectedCheckpoint(selectedValue); // Remove this checkpoint from the selected_checkpoints field
        };
        deleteCell.appendChild(deleteButton);

        // Clear the selected option and remove it from the dropdown
        selectBox.remove(selectBox.selectedIndex);

        // Update the dropdown message
        updateDropdownMessage();

        // Add the selected checkpoint ID to the selectedCheckpointIds array
        selectedCheckpointIds.push(selectedValue);

        // Update the hidden input field with the selected checkpoint IDs
        document.getElementById("selected_checkpoints").value = selectedCheckpointIds.join(",");

        // Add the selected checkpoint ID to the selected_checkpoints field
        addSelectedCheckpoint(selectedValue);
    }
}




    function addOptionToSelect(value, text) {
        var selectBox = document.getElementById("checkpoint-select");
        var option = document.createElement("option");
        option.value = value;
        option.text = text;
        selectBox.appendChild(option);

        // Enable the dropdown when an option is added
        selectBox.disabled = false;

        // Update the dropdown message
        updateDropdownMessage();
    }

    function deleteRow(row) {
        var tableBody = document.querySelector("#selected-checkpoints-table tbody");
        tableBody.removeChild(row);
        updateSno(); // Update the Sr.No. after deletion

        // Enable the dropdown when a row is deleted
        var selectBox = document.getElementById("checkpoint-select");
        selectBox.disabled = false;

        // Update the dropdown message
        updateDropdownMessage();
    }

    function updateSno() {
        // Update the Sr.No. for all rows
        var tableBody = document.querySelector("#selected-checkpoints-table tbody");
        var rows = tableBody.rows;
        for (var i = 0; i < rows.length; i++) {
            rows[i].cells[0].textContent = i + 1;
        }
        rowCount = rows.length + 1; // Update the rowCount
    }


    function updateDropdownMessage() {
        var selectBox = document.getElementById("checkpoint-select");
        if (selectBox.options.length === 0) {
            selectBox.innerHTML = '<option value="">No checkpoints available</option>';
            selectBox.disabled = true;
        } else {
            // Remove the "No checkpoints available" option if it exists
            var noCheckpointOption = selectBox.querySelector('option[value=""]');
            if (noCheckpointOption) {
                selectBox.removeChild(noCheckpointOption);
            }
            selectBox.disabled = false;
        }
    }

my laravel code looks something like this

 public function store(Request $request)
{
    // Validate the form data
    $validatedData = $request->validate([
        'guard' => 'required',
        'shiftstartTime' => 'required',
        'shiftendTime' => 'required',
        'selected_checkpoints' => 'required|array', // Ensure selected_checkpoints is an array
        'selected_checkpoints.*' => 'exists:checkpoints,id', // Validate that each checkpoint exists in the "checkpoints" table
    ]);



    // Create a new round and save it
    $round = new Round();
    $round->guard_id = $validatedData['guard'];
    $round->shiftstartTime = $validatedData['shiftstartTime'];
    $round->shiftendTime = $validatedData['shiftendTime'];
    $round->save();

    // Attach selected checkpoints to the round
    $round->checkpoints()->attach($validatedData['selected_checkpoints']);

    // Redirect or return a response as needed

Errors

    im getting the error like this how to resolve this while except checkpoint field all my fields are storing in the db but the checkpointsdata is not storing
    Im getting the error like this  rounds  rounds  
    3 / 4 requests
    3.8 kB / 3.8 kB transferred
    {message: "The selected checkpoints must be an array.",…}
    errors
    : 
      {selected_checkpoints: ["The selected checkpoints must be an array."]}
message:"The selected checkpoints must be an array."