Data binding into the listbox but not showing while I press the listbox button

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C#

Javascript

Ajax

jQuery

I'm trying to get a list box using multiselect in BS5
while running the code the button is not working
if i check in inspect the data is perfectly falling inside it but don't know what the problem is

What I have tried:

<div class="col-sm-3">
                               <div class="form-group">
                                   @Html.Label("Customer ID", new { @class = "col-md-4 form-control-label" })
                                   <select name="ddl_CustID" id="ddl_CustID" class="form-control" multiple></select>
                                   @*@Html.ListBoxFor(model => model.fld_cust_id, new SelectList("", "Value", "Text"), new { @id = "ddl_CustID", @class = "form-control" })*@

                               </div>
                           </div>

$.ajax({
                   type: "GET",
                   url: '@Url.Action("GetProvCommReport", "Report")',
                   contentType: 'application/json',
                   dataType: "json",
                   //getLocationBy: "stateid"
                   error: function () {
                       var notify = $.notify('Error Error for reading location info. Please contact administrator.!', {
                           allow_dismiss: true,
                           type: 'danger'
                       });
                       return false;
                   },
                   success: function (d, status) {
                       if (status == 'success') {
                           //var obj1 = JSON.stringify(d);
                           //console.log(obj1);
                           //var obj = JSON.parse(d);
                           var options = "";
                           var duplicate = [];
                           $.each(d, function (k, v) {
                               options += "<option value='" + v["fld_cust_id"] + "'>" + v["fld_cust_id"] + "</option>";
                               console.log(v["fld_cust_id"]);
                               duplicate.push(v["fld_cust_id"]);
                           });
                           console.log(options);
                           $('#ddl_CustID').append(options);

                           $('#ddl_CustID').multiselect({ includeSelectAllOption: true, buttonWidth: '300px', selectAllValue: 0, enableFiltering: true, maxHeight: 350, dropRight: false, enableCaseInsensitiveFiltering: true });
                           $('#ddl_CustID').multiselect('refresh')
                           $('#ddl_CustID').multiselect({
                               allselectedtext: 'all',
                               multiple: true,
                               numberdisplayed: 8,
                               buttonwidth: '100%',
                               includeselectalloption: true,
                               enablefiltering: true,
                               maxheight: 150,
                               enablecaseinsensitivefiltering: true,
                               nonSelectedText: 'Select Customer ID',
                           });

                       } else {
                           var notify = $.notify('Error Error for reading location info. Please contact administrator.!', {
                               allow_dismiss: true,
                               type: 'danger'
                           });
                           return false;
                       }
                   }
               });