The Apple TV app on iPad should be signed in automatically with the Apple Account as used to set up and register the device. It is possible to sign out and sign in within the app itself, but most users would never need to do that.
The Apple TV app on iPad would be up-to-date automatically with the iPadOS version on the device. Even if not the latest version, it should keep working for all still-supported models.
Are you trying to watch on the iPad itself, or on a different device?
I can’t replicate your issue on my iPad. Can you screenshot the error message, to see if we may find additional clues in that screen?