On change event in dropdownlist ASP.NET

GET http://localhost:9421/Products/GetDis?discount=1&productName=apple 500 (Internal Server Error)

@foreach (var item in Model) {
    <tr>
        <td>
            @Html.DisplayFor(modelItem => item.ProductName)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.ProductPrice, new { @id = "prisValue" })
        </td>
       
            <td>

               @Html.DropDownList("Status", new List<SelectListItem>
                 {
                    new SelectListItem{ Text="Student Discount", Value = "1" },
                    new SelectListItem{ Text="Loyalty Discount", Value = "0" }
                 }, "Please select discount", new { onchange = "ChangeDis(this,'" + item.ProductName + "')" })
                
            </td>

        <td>
            @Html.ActionLink("Edit", "Edit", new { id=item.Id }) |
            @Html.ActionLink("Details", "Details", new { id=item.Id }) |
            @Html.ActionLink("Delete", "Delete", new { id=item.Id })
        </td>
    </tr>
}

</table>
@section scripts{
    <script>
        
        function ChangeDis(o, proName, targetPrice) {
            // Get the triggering element
           // var element = event.srcElement;
            var targetPrice= $("#prisValue").val();
            //var targetPrice = $(element).closest('td').prev('td');
            $.ajax({
                url: '@Url.Action("GetDis")',
                data: { "discount": o.value, "productName": proName , "tPrice":targetPrice},
                success: function (data) {
                    $(targetPrice).text(data);
                }
            });
        }
    </script>
}

public ActionResult Index()
       {
           Customers customer = new Customers();

           return View(db.Products.ToList());
       }
       public JsonResult GetDis(string discount, string productName, decimal tPrice)
       {
           Customers customer = new Customers();

           var products = db.Products.ToList();

           decimal price;

           if (discount == "1")
           {

              customer.SetDiscountStrategy(new LoyalStudentDiscount());
               price = customer.ApplyDiscount(tPrice);
               //lblFinalPrice.Text = price.ToString("C");
           }
           if (discount == "0")
           {
               customer.SetDiscountStrategy(new LoyaltyDiscount());
               price = customer.ApplyDiscount(tPrice);
           }
           return Json(JsonRequestBehavior.AllowGet);
       }