I bought a gift card from UK and it said I should buy a US one

Gift cards are regional you con only use a UK gift card in the UK.

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards, or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can contact Apple Support to request a refund. When you contact Apple, please include the following information:

Your Apple ID. The refund amount. Your current shipping address.