That is absolutely not an official distribution file. It looks like someone’s template on how to change the username and group Plex runs as and didn’t do it correctly.
Of interest look at:
User=user
Group=group
You are having problems because there is no user named user and no group named group.
Here is from the official package.
chuck@udummy:/etc/systemd/system$ cd
chuck@udummy:~$ ls -la /lib/systemd/system/plexmediaserver.service
-rw-r--r-- 1 root root 746 Dec 21 18:37 /lib/systemd/system/plexmediaserver.service
chuck@udummy:~$ cat /lib/systemd/system/plexmediaserver.service
[Unit]
Description=Plex Media Server for Linux
After=network.target
[Service]
Environment="PLEX_MEDIA_SERVER_APPLICATION_SUPPORT_DIR=/var/lib/plexmediaserver/Library/Application Support"
Environment=PLEX_MEDIA_SERVER_HOME=/usr/lib/plexmediaserver
Environment=PLEX_MEDIA_SERVER_MAX_PLUGIN_PROCS=6
Environment=PLEX_MEDIA_SERVER_TMPDIR=/tmp
ExecStartPre=/bin/sh -c '/usr/bin/test -d "${PLEX_MEDIA_SERVER_APPLICATION_SUPPORT_DIR}" || /bin/mkdir -p "${PLEX_MEDIA_SERVER_APPLICATION_SUPPORT_DIR}"'
ExecStart=/bin/sh -c 'LD_LIBRARY_PATH=/usr/lib/plexmediaserver /usr/lib/plexmediaserver/Plex\ Media\ Server'
Type=simple
User=plex
Group=plex
Restart=on-failure
RestartSec=5
StartLimitInterval=60s
StartLimitBurst=3
[Install]
WantedBy=multi-user.target
chuck@udummy:~$ uname -a
Linux udummy 4.4.0-59-generic #80-Ubuntu SMP Fri Jan 6 17:47:47 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
chuck@udummy:~$
To fix this:
sudo rm /etc/systemd/system/plexmediaserver.servicesudo systemctl daemon-reloadsudo systemctl start plexmediaserver
