In Ubuntu, user “root” won’t show up as an options to login, which you need to adjust the Ubuntu to allow login as “root”.
Run this command below in your terminal (as normal user with sudo):
sudo passwd root
sudo sh -c 'echo "greeter-show-manual-login=true" >> /etc/lightdm/lightdm.conf'
Reboot and then you should be able to login as root using graphical user login.