Why I am getting exception "request aborted. Could not create SSL/TLS secure channel." in ASP.NET with C#?

Request was aborted.Could not create SSL/TLS secure channel.

ServicePointManager.Expect100Continue = true;
                   ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls
                      | SecurityProtocolType.Tls11
                      | SecurityProtocolType.Tls12
                      | SecurityProtocolType.Ssl3;
                   request = (HttpWebRequest)WebRequest.Create(RequestURL);
                   System.Text.UTF8Encoding encoding = new System.Text.UTF8Encoding();
                   request.Method = RequestMethod;
                   request.Accept = @"application/json";
                   request.ContentType = @"application/json";
                   Byte[] byteArray = encoding.GetBytes(jsonRequest);
                   request.ContentLength = byteArray.Length;

                   using (Stream stm = request.GetRequestStream())
                   {
                       stm.Write(byteArray, 0, byteArray.Length);
                   }
                   txtLog.Text += "Transact_Posting_Core()-Getting Request Stream Process Completed" + Environment.NewLine;
               }
               catch (Exception ex)
               {

                   return false;
               }




                   using (StreamReader responseReader = new StreamReader(request.GetResponse().GetResponseStream()))
                   {
                       jsonResponse = responseReader.ReadToEnd();
                       ResponseReceivedDate = DateTime.Now;
                   }


                   if (string.IsNullOrEmpty(jsonResponse))
                   {
                       txtLog.Text += "Transact_Posting_Core()-Response Fetched from Core as:" + jsonResponse + Environment.NewLine;

                       HttpWebResponse Response = (HttpWebResponse)request.GetResponse();
                       if (Response.StatusCode == HttpStatusCode.OK)
                       {
                           HTTP_StatusCode = "200";
                           ResponseStatus = "S";

                       }