My apple tv on my smart tv keeps sayingg how to watch

Unfortunately there is currently no support for in-app financial transactions for the Apple TV app on Fire TV. You will need a secondary supported other device (e.g. iPhone) in order to buy, rent, or subscribe. You can also subscribe to Apple TV+ in a web browser. Once that step is completed, then you can watch the paid-for content on any of your devices with the Apple TV app, including Fire TV.

Footnote on: Get the Apple TV app or Apple TV+ app on your smart TV or streaming device - Apple Support

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.

Note that the Apple TV app also features others content besides Apple TV+ catalog content. That other content is not included in the Apple TV+ 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 [1] [2], by subscription;
• other premium Apple TV app Channels, each by subscription.