Why can’t I submit my App Store and iTunes gift card?

If this is a newly created Apple ID, it requires a payment method to verify and make the Apple ID usable in the stores.

If this is a pre-existing account but she tried to buy something from the store and it was unable to be charged before she got the gift card, it will require a payment method to be reverified, and allow access to the stores again.

In either case, once the payment method is verified, it can be removed, and should not ask for it again.

You can add yours, just to complete verification, and then remove it once its complete.

During verification, Apple may place an authorization hold on the payment method. It should be reversed shortly after the verification process is completed.

See here: Change, add, or remove Apple ID payment methods - Apple Support

and here: About payment card authorization holds from the App Store or iTunes Store - Apple Support

You need to tell us why not. What happens when you try to use it? Are you trying to redeem it?

You need to explain exactly what is going on.

Start here: Redeem App Store & iTunes Gift Cards and content codes - Apple Support

then here if you are getting a specific error: If you can't redeem your App Store & iTunes Gift Card, Apple Music Gift Card, or content code - Apple Support

gave my daughter a $15 itunes gift card. itunes Balance shows on her apple id account. She doesnt have a payment

(credit or debit card) b/c it is HER iPad and shes only 15. When she tries to use the funds from itunes there is an error message saying you must verify payment. Can’t she just use the $15 balance from itunes w/o having to enter a debit or credit card?

We don't know.

What happens when you try and redeem it?

nope