Can’t add my Apple gift card card balance to my wallet

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

Apple gift cards are for the purchase of things from Apple only.

Why not?

What happens when you try?

Adding the Apple Account balance as a card to Apple Wallet is not supported in all countries and regions. Make sure your account is set to a region where it is supported.

click here ➜ Check your Apple Account balance in Wallet on iPhone - Apple Support

If you are getting an error when trying to add it, it probably means the option is not supported in your country.