Why I can’t the items in the database using php and mysql

I am using php and mysql for my database but unfortunately I am encountering an error and it shows this error

DataTables warning: table id=example – Invalid JSON response. For more information about this error, please see http://datatables.net/tn/1

it seems that it has an error on the $sql = “SELECT * FROM user WHERE user_type =’admin”;

$output= array();
$sql = "SELECT * FROM user WHERE user_type ='admin";

$totalQuery = mysqli_query($con,$sql);
$total_all_rows = mysqli_num_rows($totalQuery);

$columns = array(
    0 => 'id',
    1 => 'username',
    2 => 'email',
    3 => 'mobile',
    4 => 'city',
    5 => 'password',
);

if(isset($_POST['search']['value']))
{
    $search_value = $_POST['search']['value'];
    $sql .= " WHERE username like '%".$search_value."%'";
    $sql .= " OR email like '%".$search_value."%'";
    $sql .= " OR mobile like '%".$search_value."%'";
    $sql .= " OR city like '%".$search_value."%'";
}

if(isset($_POST['order']))
{
    $column_name = $_POST['order'][0]['column'];
    $order = $_POST['order'][0]['dir'];
    $sql .= " ORDER BY ".$columns[$column_name]." ".$order."";
}
else
{
    $sql .= " ORDER BY id desc";
}

if($_POST['length'] != -1)
{
    $start = $_POST['start'];
    $length = $_POST['length'];
    $sql .= " LIMIT  ".$start.", ".$length;
}   

$query = mysqli_query($con,$sql);
$count_rows = mysqli_num_rows($query);
$data = array();
while($row = mysqli_fetch_assoc($query))
{
    $sub_array = array();
    $sub_array[] = $row['id'];
    $sub_array[] = $row['username'];
    $sub_array[] = $row['email'];
    $sub_array[] = $row['mobile'];
    $sub_array[] = $row['city'];
    $sub_array[] = $row['password'];
    $sub_array[] = '<a href="javascript:void();" data-id="'.$row['id'].'"  class="btn btn-info btn-sm editbtn" >Edit</a>  <a href="javascript:void();" data-id="'.$row['id'].'"  class="btn btn-danger btn-sm deleteBtn" >Delete</a>';
    $data[] = $sub_array;
}

$output = array(
    'draw'=> intval($_POST['draw']),
    'recordsTotal' =>$count_rows ,
    'recordsFiltered'=>   $total_all_rows,
    'data'=>$data,
);
echo  json_encode($output);

$(document).ready(function() {
                          $('#example').DataTable({
                            "fnCreatedRow": function(nRow, aData, iDataIndex) {
                              $(nRow).attr('id', aData[0]);
                            },
                            'serverSide': 'true',
                            'processing': 'true',
                            'paging': 'true',
                            'order': [],
                            'ajax': {
                              'url': 'fetch_data.php',
                              'type': 'post',
                            },
                            "aoColumnDefs": [{
                                "bSortable": false,
                                "aTargets": [5]
                              },

                            ]
                          });
                        });

i am expecting to show the data in the database in a table form