Gift card has invalid code
Gift card $50 code is invalid
iPhone SE, iOS 16
Please review the steps in the support article I linked below. Multiple solutions are offered, but if none work for your specific issue use the links in the article to contact Apple Support. Apple Support Community is a public forum and we do not have access to your personal or financial information.
If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support