Why can't I buy a movie with my apple gift card?

It is possible based on Related Article -> What you can buy with your Apple Gift Card or Apple Account balance

• Buy music, movies, and more from the iTunes Store, Apple TV app, or Apple Books.

When redeeming a gift card, the amount will be credited to your (1) Apple ID balance and (2) you need to set your Apple ID balance as your default payment.

1.

See -> Check your Apple ID balance

Check your Apple ID balance – Apple Support (UK)

(2)

Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

If you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.