I tried login to a function but unexpectedly unable to login due to error.

try
            {
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
                request.Method = apiCallMethod;
                request.ContentType = "application/json";
                request.Accept = "en_US";
                request.Headers.Add("X-Api-Version:" + xApiVersion);
                var temp = "";
                //request.Headers.Add("X-Api-Version:" + xApiVersion);
                //request.Headers.Add("auth:" + sessionID);
                //if (apiCallMethod == "POST")
                //{
                //    using (var streamWriter = new StreamWriter(request.GetRequestStream()))
                //    {
                //        streamWriter.Write(json);
                //        streamWriter.Flush();
                //        streamWriter.Close();
                //    }
                //}
                ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;
                request.Timeout = 300;
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                string result;

HttpWebResponse response = (HttpWebResponse)request.GetResponse();