I buyed a gift card that i can't use

See this Apple article:  If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

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

check. ⤵️

Card is not valid

Card has already been redeemed

Card is not activated

Code is unreadable

Code must be redeemed in a different country or region

Contact Apple Support -> Contact Apple Support.

Hi jurgen,

Gift cards are country specific. If your account is set to Greece you cannot redeem an Italian card.

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

"Code must be redeemed in a different country or region

If you see a message that says the code must be redeemed in another storefront, the card was probably purchased in another country or region. You can't redeem Apple Gift Cards or App Store & iTunes Gift Cards outside of the country or region of purchase. For example, if you bought an App Store & iTunes Gift Card

in France, you can't redeem it in the App Store in the United States.

If you need more help, contact Apple Support."

By all means you can try to contact Apple as invited but usually the solution is to find somebody in the country where the card was purchased who can use that card. The other solution would be to change your country settings but that may have other requirements which means this would not be practicable.