Why Is Add funds to apple id not showing up?

See Add funds to your Apple ID https://support.apple.com/en-us/HT208687

"Before you can add funds to your Apple ID, you must have a valid payment method on file. Learn about the payment methods that you can use with your Apple ID. ... See how to Add funds to your Apple ID https://support.apple.com/en-us/HT208687  Note that "after you authorize the payment, the funds ... [are credited] to your Apple ID. Then, you can use the credit to make purchases from the App Store or iTunes store. Need more help? Contact Apple support for help here: Get started."

See if anything here helps. '

Gift cards redeem.    

Gift Card - can’t redeem.    

See the steps in the article below to check.

Change, add, or remove Apple ID payment methods - Apple Support

Fund transfers can take up to 48 hours to show up.

Have you tried signing out of the App Store/iTunes, restarting, and signing back in to see if that refreshes your balance?