Checking gift card balance

The only way to check the balance on the Gift Card is by redeeming it to your Apple account. When you say it does not work, what message are you seeing? Here is a support article for many messages you may see and what to do.

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

If you’ve not already resolved your gift card issue, these Apple Support articles may help.

Check your gift card balance - Apple Support

Check your Apple Account balance – Apple Support