G Suite users have to contend with a new disappointment every few weeks. No sooner had they learned of one pointless limitation on their account than they discover a new one. The latest blow is that Assistant says it doesn't support reminders for G Suite accounts now... even though it did just a few days ago.
If your Google Assistant is set to use a G Suite account — be it a grandfathered free account from back when Google allowed families and small companies to have them, or a company-issued paid account — you'll notice that any question to Assistant about reminders will result in the same "not available" message. That's made even more annoying by the fact that Google decoupled reminders from its search app and is now forcing all add/edit functions to go through Assistant.
As a G Suite user, you have a few workarounds, though there's no telling if they'll be functional for long. You can use Google search to type or say "remind me to [do-action] on [date/time]" and it should be processed and added to your reminders... well, as long as reminders work in the web interface (they've been known to die every few months). You can also revert the Google app to an older version and use the previous native reminder UI that processed all requests without passing by Assistant. And finally, you can add reminders from the Calendar, though the latter didn't result in those reminders syncing to and showing in the Google app for me — your mileage may vary.
Regardless of the workaround, you can't just simply ask Assistant about your reminders anymore, even though Google was very happy about adding more Assistant functionality for G Suite users a few months ago. Whether this reminder limitation is a momentary bug or a "feature," only time will tell.