Apple TV on iPhone 12 pro

It appears that your device is not on Wi-Fi, and that the user has not (yet) allowed Apple TV app to use cellular data.

Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

View or change cellular data settings on iPhone - Apple Support

There is another separate setting for Cellular, where restrictions can be set per app. Set it to On for the TV app, if your don’t mind streaming content using cellular data.