Hi, and Welcome to the Apple Community.
It seems not all Apple media and services can be purchased with gift cards.
Wondering what type of gift card you added. (There are a few different ones).
What type of gift card do I have? – Apple Support
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There are a few articles(links below) from Apple Support that may be helpful.
How to: Check your gift card balance – Apple Support
If your gift card works with you Apple ID:
What you can buy with your Apple Gift Card or Apple Account balance – Apple Support
You can manage the order of your payment methods.
(By default Apple uses the one at the top of the list.)
Change, add, or remove Apple ID payment methods - Apple Support
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If you want you can report a problem to Apple:
https://reportaproblem.apple.com/?s=6
or
You could try reaching out to the Billing support team:
There is a "Get Support" link at the bottom, on their website (link below)
Subscriptions and Billing - Official Apple Support
All the best :-)