I can’t use apple gift card

You have to redeem Apple gift cards in order to be able to use them. This adds the balance to your account where it is then automatically used when you buy something. The article "What type of gift card do I have?" explains the different kinds of gift cards Apple uses, has links on how to redeem them, and what can be purchased with each kind. Click on the link --> What type of gift card do I have? - Apple Support

If you are having problems redeeming a gift card, review the possible reasons and suggestions listed in this support article --> If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

If your issue is not listed, follow the directions at the end for contacting Apple Support.