You can have a credit card / debit card on file and an Apple Account balance.
Your default payment method will be used first by Apple when you need to make a payment.
If you have set your Apple Account balance as your default payment method and you have a CC / debit card, you pay from your default payment method.
If you have 2 CC, and you have set one CC as your default payment method that will be charge by Apple.
Sign in to your Apple ID account page and edit / update your Apple ID details -> https://appleid.apple.com
- On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.
or
If you need to change or update your Apple ID payment method
If you need to change or update your Apple ID payment method - Apple Support
- If you want to change your card, you need to add first the new card, then remove the old card.
or
Add a payment method to your Apple ID
Add a payment method to your Apple ID – Apple Support (UK)
- If you want to change your card, you need to add first the new card, then remove the old card.
See also the following Apple articles:
Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)
fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card.
If your card is not accepted as a paymenf method, you need to contact your bank.
If you have followed all the steps and instructions and if you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.