Send them the gift of apps, books, TV shows, 50 million songs, and more. Cards can be used on the iTunes Store, the App Store, and Apple Books, and toward Apple Music subscriptions. App Store & iTunes Gift Cards are issued and managed by Apple Value Services (“Issuer”).
Itunes Gift Card Exchange Machine
I also encountered a problem trying to get a iTunes $20 card to be accepted by iTunes Store. I bought the card in Sydney just before Christmas and it was a new format compared to some of the cards I had bought earlier in 2006. There was a number printed on the back of the card that said 'activation code' but that didn't work for me.
In the end I found I could peel off a sticker on the back of the card that revealed another code and it was successfulluy accepted by iTunes Store. So my advice would be to check if you can't find a code under a sticker on the back of the card.