I set up a Guest/Private Area on my new Nokia 7.1, as I use the Microsoft Launcher by default, this would allow me to look at the Pure Google theme of Android One on the Guest account.
After doing this my Google Pay did not work when went to the store (Embarrassing). After checking on this it seems that if the Guest/Private Area does not have a password, Google pay will not work and reports an error, probably as it cannot request your logon if requested by the payment terminal.
I cannot really test this out again, so if you do have a similar issue, first to check is the password is set on the Guest/Private Area accounts. Please comment below if this tip helps.