Oh darn ... I borked the syntax. Should be DISM /Online NOT DISM /Image:C: \Online
First do this: DISM /Online /Cleanup-Image /CheckHealth
Then Try this: DISM /Online /Cleanup-Image /RestoreHealth
The "Online" modifier tells the command that you are using the active image. It does not need a disk identifier. My bad.
If your system image is mounted on some other disk then the Image specifier is used but that's usually the offline option.
Also, you should open a command shell or powershell with administrative privileges.
Sorry for the mistake, I'll edit my original post as well