The Apple TV app can access more than Apple TV+ subscription content alone. That other content will have to be paid for separately. Perhaps you assumed the show or movie was part of Apple TV+, when it was not? The Up Next queue list allows adding titles that have not been purchased yet, or from services that have not been subscribed to yet. The How to Watch text was supposed to convey this situation. You may recognize the Apple TV+ content from the logo in the corner of tiles, or by filtering through the use of the tv+ tab in the app.
With the Apple TV app on your smart TV, you can access:
Unfortunately, not all platforms allow in-app financial transactions at this time. That means that for those platforms, the subscribing or buying or renting part will have to be done on a different supported device (e.g. iPhone). After that, the content will be available on your initial device (TV) as well, from linking to the Apple ID account.
2. On Amazon Fire TV, Android TV, and Sony TV devices, you can't purchase content from the Apple TV app. You can buy, rent, or subscribe on another supported device and then watch in the Apple TV app on your Amazon Fire TV, Android TV, or Sony TV device. [2]