To remove a card from Apple Pay and add a new one:
Follow the instructions here:
- Manage the cards that you use with Apple Pay - Apple Support
- Set up Apple Pay - Apple Support
- Get help adding cards to Apple Pay - Apple Support
To change your Apple ID (including iTunes Store) payment method:
Follow the instructions here including checking that your address details fully match those on file with your card issuer / bank:
The payment method that you are trying to use may not be a supported payment method.
If you still need help, then contact iTunes Support:
(I'm afraid that no-one here can assist you directly. This is a user-based technical support community - you are not talking to Apple here).