Getting Error on Calling Sevice From MonoDevelop

WebClient objCTSServiceClient = new WebClient(); objCTSServiceClient.Headers[HttpRequestHeader.ContentType] = "text/json"; ServicePointManager.ServerCertificateValidationCallback = delegate (object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors) { return true; }; ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12; result = objCTSServiceClient.UploadString(URL + "/FunctionName, "POST", Data);Calling a Service like this and Getting Error : SendFailure (The authentication or decryption has failed.) from monoDevelop C#