payment method
I need to add a new payment method because my old card is no more - it won't let me because I first need to verify the old one. I can't because it's gone...
iPhone 6, iOS 12
Try adding and removing the cards via https://appleid.apple.com/account/manage
If that also doesn't work contact iTunes Support and see if they can help you do it : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)