Paypal url return FAIL after order complete in Mvc 4

Question

Paypal return id comes fail.what should i do?? I have tried a lot..but not working..

 

public string Cart_ConfirmOrder(string UnitId)
           {
               int Quantity = 0;
               string name = "";
               try
               {
   Session["Quantity"] = Quantity;
                   Session["UnitID"] = name;
                   Session["payamount"] = ConfigurationManager.AppSettings["payamount"].ToString();
                   //Session["payamount"] = AmountCount.ToString();
                   //Session["msg"] = " You Tag request has been submitted";
                   string redirecturl = "";
                   redirecturl +=
                   redirecturl += "https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_xclick&business=" + ConfigurationManager.AppSettings["BusinessEmail"].ToString();
                   ////Product Name
                   //redirecturl += "&item_name=" + model.UnitId;
                   redirecturl += "&item_name=" + name;
                   ////Payment Section
                   //redirecturl += "&amount=" + AmountCount.ToString();
                   redirecturl += "&amount=" + ConfigurationManager.AppSettings["payamount"].ToString();

                   ////Add quatity i added one only statically
                   redirecturl += "&quantity=" + Quantity;
                   ////Currency code
                   ////redirecturl += "¤cy=";
                   ////Tax charges if any, or available or using shopping cart system
                   redirecturl += "&tax=" + 0;
                   ////Success return page url
                   redirecturl += "&return=" +
                     ConfigurationManager.AppSettings["ReturnUrl"].ToString();
                   ////Failed return page url
                   redirecturl += "&cancel_return=" +
                   ConfigurationManager.AppSettings["CancelPurchaseUrl"].ToString();

                   ////return Redirect(redirecturl);

                   return redirecturl;
                   //return retval;
               }
               catch (Exception ex)
               {
                   throw ex;
               }
   }
   public ActionResult GetResponse()
               {
       string TransactionIDc = Request.QueryString.Get("tx");
                       string transactionID = "";
                       var formVals = new Dictionary<string, string>();
                       formVals.Add("cmd", "_notify-synch"); //_notify-synch_notify-validate
                       formVals.Add("at", "AO6pILVgzXKfGZ4XyTdge9Zgmio.AbNUckZNoSqVHzWaFf8gWR0BPzqP"); // this has to be adjusted
                       formVals.Add("tx", Request["tx"]);

                       string response = GetPayPalResponse(formVals, true);
                       if (response.Contains("SUCCESS"))
                       {
                           transactionID = GetPDTValue(response, "txn_id"); // txn_id //d
                           string sAmountPaid = GetPDTValue(response, "mc_gross"); // d
                           string deviceID = GetPDTValue(response, "custom"); // d
                           string payerEmail = GetPDTValue(response, "payer_email"); // d
                           string Item = GetPDTValue(response, "item_name");
                       }
                       else
                       {
                       }
        }
       string GetPayPalResponse(Dictionary<string, string> formVals, bool useSandbox)
               {

                   string paypalUrl = useSandbox ? "https://www.sandbox.paypal.com/cgi-bin/webscr"
                       : "https://www.paypal.com/cgi-bin/webscr";

                   HttpWebRequest req = (HttpWebRequest)WebRequest.Create(paypalUrl);

                   // Set values for the request back
                   req.Method = "POST";
                   req.ContentType = "application/x-www-form-urlencoded";

                   byte[] param = Request.BinaryRead(Request.ContentLength);
                   string strRequest = Encoding.ASCII.GetString(param);

                   StringBuilder sb = new StringBuilder();
                   sb.Append(strRequest);

                   foreach (string key in formVals.Keys)
                   {
                       sb.AppendFormat("&{0}={1}", key, formVals[key]);
                   }
                   strRequest += sb.ToString();
                   req.ContentLength = strRequest.Length;

                   //for proxy
                   //WebProxy proxy = new WebProxy(new Uri("http://urlort#");
                   //req.Proxy = proxy;
                   //Send the request to PayPal and get the response
                   ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
                   string response = "";
                   using (StreamWriter streamOut = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII))
                   {

                       streamOut.Write(strRequest);
                       streamOut.Close();
                       using (StreamReader streamIn = new StreamReader(req.GetResponse().GetResponseStream()))
                       {
                           response = streamIn.ReadToEnd();
                       }
                   }

                   return response;
               }
               string GetPDTValue(string pdt, string key)
               {

                   string[] keys = pdt.Split('\n');
                   string thisVal = "";
                   string thisKey = "";
                   foreach (string s in keys)
                   {
                       string[] bits = s.Split('=');
                       if (bits.Length > 1)
                       {
                           thisVal = bits[1];
                           thisKey = bits[0];
                           if (thisKey.Equals(key, StringComparison.InvariantCultureIgnoreCase))
                               break;
                       }
                   }
                   return thisVal;

               }

 

here "response" return FAIL after order complete.

I have changed the IPN notification setting in paypal also