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, the amount will be credited to your (1) Apple Account balance and (2) 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.
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.
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.
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 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)
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
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.
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I want to get back my money.
It is not possible, see -> Check your Apple Account balance -> Check your Apple Account balance – Apple Support (UK)
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If you have an Apple Account balance that you can’t use
You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows the refund of redeemed gift balances, you can contact Apple Support to request a refund.
When you contact Apple, include the following information:
• Your Apple ID.
• The refund amount.
• Your current delivery address.