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 none of those resolve the issue, refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support
Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support