From the support document:
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
If all that has been verified, then contact Apple support as advised in the support document:
Contact Apple Support
click here-> Contact Apple Support.
You might need to provide:
- An image of the full front and back of your card.
- A sales receipt showing when your card was purchased.
- The serial number from your card.
This is a user to user forum. This is not Apple Support. If the card cannot be redeemed you need to contact Apple support directly as explained un the support document you posted from.
