I had the same issue with my install yesterday.
To fix that issue I added a user, gave the user root privilege. Then connected a HDMI monitor to the HDMI-1 connector. I then added a USB keyboard. This lets you login to the now headed unit and remove the openmediavault-md plugin.
To show all plugins do this:
sudo dpkg -l | grep openmediavault.
Then I removed the md (multidisk) plugin do this:
sudo apt-get remove openmediavault-md (this is the Raid menu item)
Once it was removed you can go to the GUI and add the md plugin or since you are at the shell you can do
sudo apt-get install openmediavault-md
Your reward is that now the Raid option will appear in the gui menu.
Sorry this is a bit of linux command line stuff but for some reason there is no GUI way to remove a plugin and that is what needed to be done on my install to get Raid to show up.
Now I am stuck on Zfs which does not work due to a missing component.