I can't use Apple TV+ on my iPad or TV

The subscription is for an Apple ID account. Verify that the same account is used on all devices.

Apple TV+ may not be available on old iPads, as it requires the modern Apple TV app (iOS 12.3 or later, I believe).

Apple TV+ may not be available on old Apple TV boxes, as it requires the modern Apple TV app (system software 7.3 or later, or tvOS 12.3 or later).

So what are the model iPad and Apple TV box?

Identify your iPad model - Apple Support

Identify your Apple TV model - Apple Support

Yes you can, you have to be logged in with the same Apple ID as your other devices. If you cannot figure it out, please give AppleCare a call. AppleCare contact Information