You can have a credit card / debit card on file and an Apple Account balance.
If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.
If you have a credit card & a debit card, and you want to use your debit card you need to set your debit card as your default payment method that will be charge by Apple.
Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.
See 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.
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.
- or if you use your Apple Account balance, you need to set your Apple Account balance as your default payment.
Add money to your Apple Account balance
Add money to your Apple Account balance - Apple Support
What you can buy with your Apple Gift Card or Apple Account balance
What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)
Apple Gift Card and Apple Account balance can't be used for some purchases
- You can't use your Apple Account balance to send gifts or gift cards.
- If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.
- Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.
If you update your payment method or add funds to your Apple Account 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.
If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK)