If you can't redeem your gift card in the App Store or iTunes Store, here are some things you can try.
Apple Gift Card - App Store, iTunes, iPhone, iPad, AirPods, MacBook, accessories and more (eGift): Gift Cards
Buy UK iTunes Gift Cards Online - Email Delivery - MyGiftCardSupply
I can't add my 100$ Apple gift card to my… - Apple Community
iTunes won't process order - Apple Community
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unable to redeem my Apple Store gift card… - Apple Community
Apple Gift Card - App Store, iTunes, iPhone, iPad, AirPods, MacBook, accessories and more (eGift): Gift Cards
When i redem my apple gft card then i see… - Apple Community
Redeem iTunes Gift Card on Android Phone, apple gift cards redeem
What type of gift card do I have? - Apple Support (CA)