The remote server returned an error: (400) bad request. while paypal return url

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C#

PayPal

I am getting 400 Bad request when try to payment through the pay pal.
Here i got the transaction id but also i did not get the success response
How can i solve this error please me.

My code is:

public void Getresponse()
       {
           try
           {

               ServicePointManager.Expect100Continue = true;
               ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;
               ServicePointManager.DefaultConnectionLimit = 9999;

               string authToken = ConfigurationManager.AppSettings["PaypalToken"].ToString();
               string txToken = Request.QueryString["tx"];
               Label1.Text = txToken;
               string query = "cmd=_notify-synch&tx=" + txToken + "&at=" + authToken;
               int pid = Convert.ToInt32(Request.QueryString["Pid"]);

               //Post back to either sandbox or live
               string strPaypalLive = ConfigurationManager.AppSettings["PaypalLive"].ToString();
               HttpWebRequest req = (HttpWebRequest)WebRequest.Create(strPaypalLive);
               var encoding = new UTF8Encoding();
               //Set values for the request back
               req.Method = "POST";
               req.ContentType = "application/x-www-form-urlencoded";
               req.ContentLength = query.Length;

               //Send the request to PayPal and get the response

               StreamWriter streamOut = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII);
               streamOut.Write(query);
               streamOut.Close();
               StreamReader streamIn = new StreamReader(req.GetResponse().GetResponseStream());
               string strResponse = streamIn.ReadToEnd();
               streamIn.Close();

               Dictionary<string, string> results = new Dictionary<string, string>();
               Label3.Text = strResponse;
               if (strResponse != "")
               {
                   Label2.Text = strResponse;
                   StringReader reader = new StringReader(strResponse);
                   string line = reader.ReadLine();

                   if (line == "SUCCESS")
                   {
                       Label2.Text = "Inside Success !";
                       ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "msg", "alert('Hello')", true);
                       using (OwlEducationEntities context = new OwlEducationEntities())
                       {
                           if (Request.QueryString["Custom"] != null)
                           {
                               ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "msg", "alert("+txToken+")", true);

                               string QryString = Request.QueryString["Custom"].ToString();
                               GRI_Package pkg = (from t in context.GRI_Package where t.PackageID == 3 select t).SingleOrDefault();
                               pkg.TransectionID = txToken;
                               pkg.IsPaid = true;
                               context.SaveChanges();
                           }
                       }
                       //if (Request.QueryString["Custom"] != null)
                       //{
                       //    string QryString = Request.QueryString["Custom"].ToString();
                       //}

                   }
                   else if (line == "FAIL")
                   {
                       Label3.Text = "Fail"+line;
                       // Log for manual investigation
                       //Response.Write("Unable to retrive transaction detail");
                   }
               }
               else
               {
                   //unknown error
                   Label3.Text = "Error";
                   Response.Write("ERROR");
               }
           }
           catch (Exception Ex)
           {
               Label3.Text ="catch"+ Ex.Message;
               Exceptions.LogException(Ex);
           }

}

What I have tried:

i tried this code for getting response

StreamWriter streamOut = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII);
streamOut.Write(query);
streamOut.Close();
StreamReader streamIn = new StreamReader(req.GetResponse().GetResponseStream());
string strResponse = streamIn.ReadToEnd();
streamIn.Close();