I can’t pay anything with my Apple gift cards

You should be aware that when you make purchases, for example in the App Store, the payment will "look as if" it is being deducted from your payment card. But if you have money on your Apple Balance, then the amount will always be paid from it first, before your primary payment method.

If you still experience problems paying with your Apple Balance, contact Apple Support:

Contact Apple Support here:

Contact Apple for support and service - Apple Support

See How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (AU)