Thanks Jay! I unfortunately lost this Discourse install due to, well, foolishness on my part (mishandling a virtual machine). So I can’t go back and check, nor do your recommended troubleshooting, which I feel pretty bad/dumb about. But there we have it.
What I can tell you is that I followed the Discourse Ubuntu setup instructions:
So it’d be whatever version of Postgres is installed that way. And I had tried to import a couple of times by this point, without clearing the DB in-between, so maybe that has something to do with it.
I followed the Ubuntu dev environment method rather than use the Docker approach because when I’d tried that earlier (on Digital Ocean), none of the instructions I was finding for how to do, well, anything were working because, presumably, it was inside Docker/containers and required a different approach.
I am very much operating at the limits of my understanding here, unfortunately, but it felt worth at least trying to do myself. With this big setback, and the knowledge that there are likely more troubleshooting steps ahead, I think it’s very likely time to seek professional help. So I’m off to try to get some budget for that. 
Thanks again for your help!