This error message appears when there are issues authenticating via the login method your Guardian Tales account is linked to. It may be possible to resolve the issue by performing a cache clear (instructions here), but in most cases, you’ll need to reauthenticate with your linked login service (log out and log in again).
You’ll find detailed instructions for how to do that below. Please be aware that signing out of any of the below services will also impact any other accounts or device features they’re associated with.