By default in the ubuntu installation root user is disabled.

Fire the command from installed user which has super privilage or created while installing the ubuntu

Enabling the root account

sudo passwd

Sudo will ask for your password then prompt for root password two times as :

[sudo] password for username: (enter own password)
Enter new UNIX password: (enter a new password for root user)
Retype new UNIX password: (enter 2nd time same password for root user)
passwd: password updated successfully

Now the root user is enabled.

Disable the root account

Run the following command to disable the root user in ubuntu

sudo passwd -l root