Why my card can’t be redeemed
Why my Apple gift card can’t redeemed
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Try signing out of your Apple ID and then back in again in case the balance has been applied to your account but isn't showing correctly.
If you bought an iTunes gift card voucher which you cannot redeem then you should first try If you can't redeem your App Store & iTunes Gift Card, Apple Music Gift Card, or content code - Apple Support in case you are either entering the code incorrectly, or the card wasn't activated. For activation issues seek redress from the vendor.
If you have sent the card details to someone else as a form of remote payment, or have bought a card that has already been redeemed, then you have been scammed. See iTunes Gift Card Scams - Official Apple Support for more information.
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