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:
- 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.
- 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.
- 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
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.