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.
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 -> How to redeem your Apple Gift Card or App Store & iTunes gift card
How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)
When redeeming a gift card, (1) the amount will be credited to your Apple Account balance and (2) then you need to set your Apple Account balance as your default payment.
(1) See -> Check your Apple Account balance
Check your Apple Account balance – Apple Support (UK)
(2) See -> Add a payment method to your Apple ID
Add a payment method to your Apple ID – Apple Support (UK)
If you already use your Apple Account balance, the Apple Gift Card or App Store & iTunes gift card is automatically added to your Apple Account balance.
See also -> 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)
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- 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 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)
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If you have a valid payment method on file, instead of buying or order an App Store & iTunes gift card, you can Add money to your Apple Account balance anytime, no gift card needed, see -> Apple Gift Card - Apple (UK)
See also: If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card
If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support
Beware of -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support