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

string RequestURL = txtRequestURL.Text;
              string jsonRequest = txtRequest.Text;

              txtLog.Text += "Transact_Posting_Core()-Creating/Assigning HTTP Web Request Process Started with Request URL:" + RequestURL + ",Request:" + jsonRequest + Environment.NewLine;

                  //System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
                  ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls
                    | SecurityProtocolType.Tls11
                    | SecurityProtocolType.Tls12
                    | SecurityProtocolType.Ssl3;


              HttpWebRequest request = (HttpWebRequest)WebRequest.Create(RequestURL);
              System.Text.UTF8Encoding encoding = new System.Text.UTF8Encoding();
              //request.Host = "";
              request.Method = "POST";
              request.Accept = @"application/json";
              request.ContentType = @"application/json";
              Byte[] byteArray = encoding.GetBytes(jsonRequest);
              request.ContentLength = byteArray.Length;
              //request.Proxy = null;
             //string isProxyReqd = ConfigurationManager.AppSettings["isProxyReqd"] != null ? ConfigurationManager.AppSettings["isProxyReqd"].ToString() : string.Empty;
             // if (isProxyReqd == "1")
             // {
             //     request.Proxy = new WebProxy();
             // }

              request.ProtocolVersion = HttpVersion.Version10;

              string IsRequestHeaderFlag = ConfigurationManager.AppSettings["IsRequestHeaderFlag"] != null ? ConfigurationManager.AppSettings["IsRequestHeaderFlag"].ToString() : string.Empty;
              if (IsRequestHeaderFlag == "1")
              {
                  request.Headers.Add("X-API-KEY", txtXAPIKey.Text);
              }
              txtLog.Text += "Transact_Posting_Core()-Started Transact Posting Request Stream Process" + Environment.NewLine;
              using (Stream stm = request.GetRequestStream())

              {
                  stm.Write(byteArray, 0, byteArray.Length);
              }
              txtLog.Text += "Transact_Posting_Core()-Completed Transact Posting Request Stream Process" + Environment.NewLine;

              txtLog.Text += "Transact_Posting_Core()-Started Transact Posting Response Process" + Environment.NewLine;
              HttpWebResponse response = (HttpWebResponse)request.GetResponse();

              StreamReader sr = new StreamReader(response.GetResponseStream());
              string result = sr.ReadToEnd();
              txtResponse.Text = result;
              txtLog.Text += "Transact_Posting_Core()-Completed Transact Posting Response Process" + Environment.NewLine;