Try this:
1.
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 ID balance and (2) then you need to set your Apple ID balance as your default payment.
(1) See -> Check your Apple ID balance
Check your Apple ID 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 update your payment method or add funds to your Apple ID 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.
2.
See this Apple article: 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
- You can't redeem Apple Gift Cards or App Store & iTunes Gift Cards outside the country or region of purchase. For example, if you bought an App Store & iTunes Gift Card in France, you can't redeem it in the App Store in the United States.
Card is not valid
Card has already been redeemed
Card is not activated
Code is unreadable
Code must be redeemed in a different country or region
Contact Apple Support -> Contact Apple Support.
See also -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support
