I want to buy a movie but it wont let me

On a few select devices, you currently cannot make financial transactions in the Apple TV app.

Get the Apple TV app or Apple TV+ app on your smart TV, streaming device, or game console - 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.

On what device is this happening?

Buying movies, or renting movies, is different from subscription services. Subscription movies often can’t be bought, and movies for purchase may not be available through subscription. You do not need any subscription to buy a movie.

When payment is done through Apple, then it will try to use the payment method as linked through your Apple ID account. For subscriptions, the payment method must additionally support automatic periodical charges. If your payment method is a card, then that card must be issued in the same region as your account region. In many regions, users may alternatively add balance to their account first, which is then deducted by fees or purchases.

Payment methods that you can use with your Apple ID (by country) - Apple Support

Change, add, or remove Apple ID payment methods - Apple Support

Change your Apple ID payment method if you don't have an Apple device - Apple Support

If your payment method is declined in the App Store or iTunes Store - Apple Support

Add money to your Apple Account balance - Apple Support

Check your Apple Account balance - Apple Support