Call failed. An error occurred while sending the request

I am getting below error while try to get the token from the,

await rc.Authorize(username, extension, password);

which throughs below exception,

{"Call failed. An error occurred while sending the request. POST https://platform.ringcentral.com/restapi/oauth/token"}

Log:

at Flurl.Http.FlurlRequest.<HandleExceptionAsync>d__23.MoveNext()

at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)

at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)

at Flurl.Http.FlurlRequest.<SendAsync>d__19.MoveNext()

at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

at Flurl.Http.FlurlRequest.<SendAsync>d__19.MoveNext()

at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)