How to create a primary iCloud email address with an existing @mac.com account?

I can't create a primary iCloud email address, despite having signed up for iCloud+.

I previously had a @mac.com email address, that is still registered on my account / linked to my apple account, but that doesn't seem to help.

I have tried logging out and back in on both my phone and laptop, but on the phone I get 'Something went wrong" and on the laptop 'Cannot complete request.'

HELP!! How do I get out of this loop?

[Re-Titled by Moderator]