Sumation Error

Question

Hello Team,

Am trying to sum up the quantity and Total Amount in my dataTable footer, but is showing underline red, how do I modified the code to to perform this sumation and the total should only display the Quantity and TotalAmount at the footer after a date range is selected such as StartDate to EndDate , 

 public ActionResult getNewOrder()
        {
            using (db = new ASPNETMASTERPOSTEntities())
            {
                var dataList = db.tblCustomers.Include("tblOrders");
                var modifiedData = dataList.AsEnumerable().SelectMany(x => x.tblOrders.Select(d => new OrderViewModel
                {
                    CustomerId = x.CustomerId,
                    CustomerName = d.tblCustomer.CustomerName,
                    PhoneNo = d.tblCustomer.PhoneNo,
                    OrderDate = d.tblCustomer.OrderDate,
                    CategoryName = d.CategoryName,
                    ProductName = d.ProductName,
                    Quantity = d.Sum(x=>x.tblOrders.Quantity),
                    UnitPrice = d.UnitPrice,
                    TotalAmount = d.Sum(x => x.tblOrders.TotalAmount),
                    OrderId = d.OrderId
                })).ToList();
                return Json(modifiedData, JsonRequestBehavior.AllowGet);
            }
        }