I’m happy to share what I’ve learned and to expand on what MovieFan.Plex said.
I have a Mac mini with Mojave 10.14.6, and I know exactly how to install PMS on it., as I’ve done it maybe 20 times in the last few months for testing. Those are step by step instructions, and it won’t be long now 
And in case you like tables (and who doesn’t? we eat on them all the time)
Plex Media Server for Mac – Version Matrix
| Mac Version | The Last Known Working Versions | Status as of 20 Nov 2022 |
|---|---|---|
| ≤ 10.14.6 | 1.28.2.6151 1.29.0.6244 | If you upgrade beyond this version, you get an error, Settings are unavailable.[1] If you fresh install beyond this, Setup will fail at the Friendly Name step.[2] |
| ≥ 11.0.0 | current | At version 1.29.2 Plex increased security on MacOS with mandatory claiming during Setup where you’re required to authenticate online.[3]
|
So now that you have the exact steps to uninstall and install, use 1.29.0 with them, and you should be up and running pretty soon.™ It’s not often on a Mac we have to be so pedantic, but here we get an opportunity to learn some tricks.
-
PMS returns HTTP Error 500 when asked for
/:/prefs?↩︎ -
mostly mandatory according to the topic on the forum, where it explains nearly everyone will install PMS on a Mac and not notice any change. That’s because during install, when you’re directed to a Plex Sign-in screen, you expect to see one. So if 95% of people don’t notice or are unaffected is it worth mentioning? I think that’s why Jesus made footnotes. ↩︎
