How to redeem your apple gift card
1.Go to your App Store.
2.Click on your Profile Icon.
3.Once you do that, scroll down and you’ll see Redeem Gift Card or Code as below
4.Enter the voucher digits and press redeem.
Errors:
1. If you got an error that your card has already been redeemed as below
you might have already redeemed it, so you must sign out of the App Store or iTunes Store on all your devices, then sign in again, then check your Apple ID balance
Learn how to check your Apple ID balance
You must redeem the code manually by entering the digits without copying and pasting.
3.If you got an error that your code is not valid as below
You must check the code you entered correctly and the same digits,if you confirmed that you added the code digits correctly, you can contact the support team.
4.If you got an error that your code must be redeemed in the US storefront as below:
You must redeem the code in US account as your account is not US.