Cannot authorize via C# Client

sample code below:

rc = new RestClient(Config.Instance.appKey, Config.Instance.appSecret, false);

var result = rc.Authorize(Config.Instance.username, Config.Instance.extension, Config.Instance.password).Result;

The error is "Request to https://platform.devtest.ringcentral.com/restapi/oauth/token failed with status code 400 (Bad Request)."

In case I try to use C# SDK, I always get following response error:

{

"error" : "invalid_grant",

"error_description" : "Invalid resource owner credentials."

}