Redeem my gift card for App Store

Go to this link - Redeem your App Store & iTunes Gift Card

Redeem a gift card with your iPhone, iPad, iPod touch

1. Peel or gently scratch off the label on the back of the gift card to reveal a 16-digit code that starts with X.
2. On your iPhone, iPad, or iPod touch, open the App Store app.
3. At the bottom of the screen, tap Today.
4. At the top of the screen, tap the sign-in button  or your photo.
5. Tap "Redeem Gift Card or Code." If you don’t see “Redeem Gift Card or Code,” sign in with your Apple ID. Learn what to do if you forgot your Apple ID.
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8. Tap "Use Camera" and follow the instructions on the screen. If you have trouble redeeming the card, tap "You can also enter your code manually," then follow the instructions on the screen. Make sure that you enter the 16-digit code that begins with X.
9. Tap Done.

Can't redeem your gift card?