The problem I was saying was that my payment was declined as unauthorised every time I tried to pay for development enrolment. I tried several cards, all of which continued to work fine for all other shops and websites but would not be accepted by Apple Developer either in the app or on the website. The solution was that I needed to first go to account.apple.com, sign in with the Apple account used for my development work, and add a payment method and shipping delivery address to my account profile. Then, I was able to return to the Apple developer enrolment workflow, select my saved payment method and complete the transaction.