Apple TV + and Amazon firestick

I’m afraid that is as intended:

1. Users on Fire TV devices and Android TV/Google TV devices would get the “How to Watch” blurb for all items that are not included in the catalog of the current subscription(s) or past buy-items for the Apple TV app. It basically says to do the financial transaction on a different device (e.g. iPhone).
2. Prison Break is not part of the Apple TV+ subscription catalog. It was a show on Fox, now part of Disney. Apple TV+ content is focussed on Apple Original content instead. It streams on Hulu, if you subscribe to that.
3. Prison Break is available under license for Apple and others to resell (per episode, or per season, or per complete series), through the Store on supported devices. Buy-to-keep, not as part of a subscription.

With the Apple TV app on your smart TV or third party streaming device, you can access:

• iTunes Store movies and TV shows (buy or rent);
• Apple TV+ channel featuring Apple Originals, by subscription;
• other select premium Apple TV app Channels, each by subscription.

➔ An Apple TV+ subscription covers just some of the content in the Apple TV app.

About Apple TV+ - Apple

Apple Originals - Apple TV+ Press

List of Apple TV+ original programming (episodic/series) - Wikipedia

List of Apple TV+ original films - Wikipedia

Apple TV+ content has its overview in the Apple TV+ tab of the Apple TV app.

Apple TV+ content is also marked with a small “tv+” logo in a corner of the tile:

If you (already) subscribe to Apple TV+, then [normally] the Apple TV+ content in the Apple TV app would be accessible after signing in, through the in-app Settings, with the Apple ID account that has this subscription linked to it.

… I cannot watch original content …

What happens instead? Do you get an error message? What does it say exactly?