401 “unauthorized_client” means that Google does not accept your attempt because the client_id and secret you are trying to post either do not correspond to the application which generated this refresh token or this token is no longer valid. Check that you are applying the right client (all .apps.googleusercontent.com id and its secret), the application is not in Test mode, or obtain a new token by executing the consent flow again, and provide the credentials one time. either through the body of the form or “Authorization” header, not both.