Logged a similar question a few days ago, on OMV 3.0. I had failed to read Openmediavault 3.x - Important Information!. So started over with 2.1!
I'm seeing the same behaviour & screenshot. This time though, no messing with partitions. A straight up install, with the operations:
* Install from ISO
* Upgrade all, from upgrade tab. My UI timedout while it was upgrading, maybe that messed up stuff?
* Install netatalk plugin (worked perfect).
* Install OMV extras from web, by downloading openmediavault-omvextrasorg_latest_all.deb
* Activate Plex plugin
* No Plex on "service" tab
* Reboot
* No Plex on "service" tab
I still only see this (plus the Apple Filling).
Still trying to think of what I missed, but can't. Hoping for quick pointers.