Dropdown is not showing the requested data

The drop down customers is not showing the list of customers.I can not finde what I’am missing. I want to get the list of customers in the first dropdown and than get the list of order numbers of the selected customer. If I manage to get beyond this, then I would like to display the contents of the order in a table. But now, unfortunately, I’m stuck here. Thanky ou if you could help me finde wher I went wrong.
My code:

index.php:

<?php
    include 'settings.php';
    $sql = "SELECT DISTINCT K_customers.Name, K_customers.Vo FROM K_customers
    INNER JOIN K_orders ON K_customers.Vo = K_orders.Vo
    WHERE K_customers.Name = '{$_POST['customers']}'";
    $result = mysqli_query($conn, $sql);
?>
<div  class="container"><br>
    <div class="row">
        <div class="col-md-3">
            <div class="form-group">
                <label for="customers">Vevő:</label>
                <select id="customers" name="customers">
                <option value="">-- Válasszon --</option>
                <?php
                    while($row = $result->fetch_assoc()) {
                        echo "<option value='".$row['Vo']."'>".$row['Name']."</option>";
                    }
                    ?>
                </select>
            </div>
        </div>
        <div class="col-md-3">
            <div class="form-group">
                <label for="orders">Rendelés:</label>
                <select id="orders" name="orders">
                    <option value="">-- Válasszon --</option>
                </select>
            </div>
        </div>
    </div>
</div>
<script>
$(document).ready(function(){
    // Vevő kiválasztása eseménykezelője
    $('#customers').change(function(){
        var customerName = $(this).val();
        if(customerName){
            // AJAX kérés az adatok lekérdezéséhez
            $.ajax({
                url: 'get_orders.php',
                type: 'post',
                data: {customerName: customerName},
                dataType: 'json',
                success:function(response){
                    // Töltse fel az options-t a vevőhöz tartozó rendelésekkel
                    $('#orders').empty();
                    $('#orders').append('<option value="">-- Válasszon --</option>');
                    $.each(response,function(index,data){
                        $('#orders').append('<option value="'+data+'">'+data+'</option>');
                    });
                }
            });
        }
        else{
            $('#orders').empty();
            $('#orders').append('<option value="">-- Válasszon --</option>');
        }
    });
});
</script>

get_orders.php:

<?php
include 'settings.php';
$vo = $_POST['customers'];
$sql = "SELECT `order` FROM `K_orders` WHERE `Vo` = ?";
$stmt = $conn->prepare($sql);
$stmt->bind_param('s', $vo);
$stmt->execute();
$result = $stmt->get_result();
$orders = array();
while($row = $result->fetch_assoc()) {
    $orders[] = $row['order'];
}
echo json_encode($orders);
?>