"Apple Pay cannot be used when automatic login is enabled." ...but automatic login is not enabled.
What it says on the tin. Automatic login is not, and has never been enabled on this machine, yet Apple Pay seems to think it is - and as a result, I can't actually set Apple Pay up.
Kinda basic stuff to get wrong, don't you think?
MacBook Pro TouchBar and Touch ID, macOS High Sierra (10.13.1), Recently upgraded to High Sierra