[MM3-users] Mailman REST API not available. Please start Mailman core.

Okay, so I got the webservice of Mailman3 up and running, but when I go to mailman3/postorius/lists/, I get the following error:

Something went wrong Mailman REST API not available. Please start Mailman core.

When I logged in to the admin user, it threw a server 500 error, but I could refresh and see myself logged in. If I try to go to lists or mailman settings, however, I get the error above.