Credit cards

Open the iTunes Store on the iPad. Select the music icon or one of the selections along the bottom.. Scroll down to the bottom of the page. You will see a box with your Apple ID in it.

Tap on the box & enter your password. You will then be able to see your account information & change/replace your payment information.

Not sure if this applies to you, but this may be the reason:

Why can’t I select None when I edit my Apple ID payment information? - Apple Support