I honestly don’t know if the error and the missing file are related. But one step at a time. Regarding the missing file:
My raspberry pi doesn’t have that file either. I think whoever made those instructions was trying to be helpful, but didn’t keep them up to date with the current release.
You shouldn’t need to change the user from ‘plex’ to ‘pi’. So don’t bother with the step you’re stuck on. Go to the “Prepare the Mount Point” portion of the instructions and run the commands again, but where they say ‘pi’, change it to ‘plex’. That should at least get you to where Plex can access your media.
Not to say you won’t run into further issues, but it should get you one step closer.
the problem is, that when I fresh install plex server and login with my plex account, the Website automaticaly goes to “home” instead to “Server” setup, so I can’t configure my rapsberry pi as a server, it just doesn’t work.