@chrishamm That was the problem!
pi@legionXY:~ $ sudo systemctl enable duetpluginservice-root
Created symlink /etc/systemd/system/multi-user.target.wants/duetpluginservice-root.service → /lib/systemd/system/duetpluginservice-root.service.
pi@legionXY:~ $ sudo systemctl start duetpluginservice-root
pi@legionXY:~ $
Success!