Change Payment Information

Removing a credit card from an account - Change or remove your Apple ID payment information - https://support.apple.com/HT201266

If you can‘t choose None when you edit your Apple ID payment information - https://support.apple.com/HT203905 - also make sure you don't have any active subscriptions, are set as the Family Organizer for a Family Sharing plan, or owe any money for past purchases.

What happens when you try? Error message(s)?

If necessary, contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option. 

iTunes Support

iTunes Support – world wide