Ubuntu disables root account by default. But you can get it back, here is how
– Open your Terminal
– Run the command
sudo passwd root
This command will ask you to enter & confirm your root password and your root account will be enabled.
– Open /etc/X11/gdm/gdm.conf and change the AllowRoot
AllowRoot = true
– Save the file, reboot or restart Gnome to activate your new setting