Matt,
The error is just above that.
PlexMediaServer install: Systemd configuration error detected:
PlexMediaServer install: File “/etc/systemd/system/plexmediaserver.service” is leftover from an earlier Debian/Ubuntu bug.
PlexMediaServer install: Please transfer any customizations (APP_DIR, User, Group, TMPDIR, and/or UMask) to a proper “/override.conf”
PlexMediaServer install: and reinstall.
You have the Unit Override file in /etc/systemd. I can’t support that because it conflicts with /lib/systemd/system/plexmediaserver.service.
What I do support is the official Service Override in /etc/systemd/system/plexmediaserver.service.d/override.conf
The installer message is telling you what to do – Take your customization variables and put them in a proper override.conf then delete the full unit override.
Details are here 
