Apple Gift Card

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

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Card is not valid

• If you see a message that says that your card is not valid, here's what to do:

1. Make sure that your card is not an Apple Store Gift Card. Apple Store Gift Cards are gray, white, silver, or gold. You can’t redeem Apple Store Gift Cards in the App Store or iTunes Store. Find out what to do if you have an Apple Store Gift Card or don't know what type of gift card you have.
2. Try entering your redemption code manually. When you redeem a gift card, tap or click "You can also enter your code manually." Make sure that you enter the redemption code that begins with "X", not a different number on the card.
3. Make sure that you enter the correct characters. Some letters and numbers look alike, such as:

• B and 8
• D and O
• E and 3
• G and 6
• O and Q
• O and 0
• S and 5
• U and V
• Z and 2

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