Need help using my gift card

Follow the steps listed in: Redeem App Store & iTunes Gift Cards and content codes - Apple Support

You can redeem a gift card or code on your device by following these simple steps: 

1. Open the App Store app. 
2. At the bottom of the screen, tap Today. 
3. At the top of the screen, tap the sign-in button  

 or your photo. 

1. Tap "Redeem Gift Card or Code." If you don’t see “Redeem Gift Card or Code,” sign in with your Apple ID. 
2. Tap "Use Camera" and follow the instructions on the screen. If you have an older card or you're in certain countries or regions, you might not be able to redeem a card with your camera. Tap "You can also enter your code manually," then follow the instructions on the screen. 

• • If you have an App Store & iTunes Gift Card, enter the 16-digit code, which starts with X, from the back of the card. 
  • If you have a promo or content code, enter the code from the back of the card. 

And you're done! 

If the article I referenced in my first post did not help try this one: If you can't redeem your App Store & iTunes Gift Card, Apple Music Gift Card, or content code - Apple Support