By default root login is disabled in Ubuntu 12.04 and newer.
Follow the steps to enable the logon:
- Open a terminal window.
- Using your favorite editor open the file with privileged rights:
sudo vim /etc/lightdm/lightdm.conf
- Append the line at the end of the file
- Set root password
sudo passwd root
- Reboot the machine and you will be able to login as root.