It would help to know how you did that. Did you wipe the server and start over? Did you just purge Mailman3 and its configuration files? Something else?
It's a bit hard to say, my brain got mushy after a few hours of staring at bash. What was happening was the install was never completing, there was always an error blocking it. One of the main ones I found was pymysql for python2 wasn't installed. Once I installed that, it started to be happy. I also had to run dpkg-reconfigure for mailman3-web and after that I had a working web service.
As far as Hyper Kitty, the web service is up (the webpage runs, at least).
And I did check systemctl status mailman3, it was inactive. So I did systemctl start mailman3 and it is now running. However, the same error still shows up on postorius.
Something went wrong Mailman REST API not available. Please start Mailman core.