Zoom add-in for Outlook (web) "Installation failed"

For a zoom account shared by several people, we established a free Outlook account, which is only used via the web interface.  It would be very convenient to add the Zoom Outlook Add-in (Web) to this account, but when following the intructions here  the Microsoft Add-in page pops up "Taking you to Microsoft 365 to complete the process" and then returns to the add-in page showing "Installation failed". No other message or hint.

Very possibly we need to do something to the Outlook account to get it authorized for add-ins? (Like pay money?) but I can't find any info on how to do that. All hints welcome!