By default, the root user is locked in Ubuntu. To unlock it, simply run the command below, and provide the new password for the root user when prompted.

sudo passwd root
[sudo] password for nik: 
Enter new UNIX password: 
Retype new UNIX password:

Now you should be able to log in as the root user using the su command:

$ su -

If for some reason you want to lock the root user password after it has been unlocked you can use:

$ sudo passwd -l root