Have you added custom oauth credentials in the past? Because there are no issues with the standard credentials provided with Big Brother that I know off, and the standard integration has been approved and everything.
Try removing (yes, removing) the
bigbrother.native_app_client_secret system settings and running a reinstall for Big Brother. That should re-create the settings with their default values and let you re-authorize.
If you do want to use custom oauth credentials note that for v2 you’ll also need to enable the
Google Analytics Admin API on the project and after 100 users your project needs to get approved by google.