Redeeming Gift Card

See information in If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

including the following:

If you see a message that says that your card has already been redeemed, you might have already redeemed it. Sign out of the App Store or iTunes Store on all of your devices, then sign back in. This refreshes your Apple Account balance. Learn how to check your Apple Account balance.

If you need more help, contact Apple Support.

This is primarily a user-to-user support forum; other Apple users like you.

If you purchased the Gift Card at a retail location where the Gift Cards are displayed out in the open where anyone can tamper with them, then don't do that. Gift Cards should only be purchased at locations that sell them securely behind a counter.