Hello Emilyrichardson22,
Thanks for using Apple Support Communities. We understand that you're seeing an error when trying to redeem a gift card. We're happy to help.
"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 ID balance. Learn how to check your Apple ID balance.
If you need more help, contact Apple Support."
If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card
Have a nice day!