Just a heads up: there is a massive, incredibly detailed [dead] comment below by luyao_street that actually explains this exact issue. It seems to be caught by the spam filter, but it's a goldmine.

The TL;DR of their advice, cheapest first:

1. Try adding the card directly to your Apple Account from an iPhone/iPad first (the web page often silently fails). 2. Use Safari (cards often fail in Firefox). 3. Stop changing variables (VPN + region switch + different browsers = triggers Apple's fraud algorithms as a 'takeover'). 4. Request a phone callback via the developer portal and ask them to complete the order manually. 5. Crucial flow check: Individuals pay at enrollment. For an Organization, you pay only after Apple verifies your info and emails you next steps. If your LLC is asking for money before that email, you are in the wrong flow. Also, for orgs, you must use a custom domain email (no gmail) and the D-U-N-S info must match character-for-character.

Might be worth scrolling down and reading it before you give up!