Added money with a gift card and can’t use them.

You may need to have a payment method, see ⤵️

Sign in to your Apple ID account page and edit your Apple ID details.

https://appleid.apple.com

On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.

Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

Have you tried signing out of the App Store/iTunes, restarting the device/computer, and then signing back in to see if you can use your balance?