When I try to install Plex Media Server I get the following message:

Failed to stop plexmediaserver.service: Unit plexmediaserver.service not loaded.
warning: plexmediaserver- Header V4 RSA/SHA1 Signature, key ID 3adca79d: NOKEY
Job for plexmediaserver.service failed because the control process exited with error code.
See "systemctl status plexmediaserver.service" and "journalctl -xe" for details.
Back to Package Manager

I have uninstalled Plex, clicked the update button, rebooted the STi5 and tried installing again with the same result. Any help is appreciated. Thanks!