Techpository

sudo passwd root
sudo sh -c 'echo "greeter-show-manual-login=true" >> /etc/lightdm/lightdm.conf'

Root won’t show up as a user, but “Login” will, which is how you manually log in with users not shown in the greeter.

Rebooted and then you should be able to login as root.

By: reverendj1 and gertvdijk