That's a problem with running the command on Windows. I suggest you open the Windows command line (cmd) and run this command on a test image:
Code:
magick convert path/to/input/image -quality 80 -colorspace gray /path/to/output/image
Take whatever error messages arising from the command to the ImageMagick forums to get help on what went wrong.
Once you get the command working on Windows command line (using cmd.exe), we would be able to investigate whatever problems arises with the plugin.