Can’t add funds to App Store
Can’t update payment because it says I have to verify old card but I no longer have that card
iPad mini, iOS 12
Try adding the new card and removing the old one 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’)