When the first time you installed Ubuntu, there is no password for root account. You have to set it first for a security reason.

You must type “sudo passwd”

hostmaster@hostmaster-desktop:~$ sudo passwd
[sudo] password for hostmaster:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

To login as root, type “su”

hostmaster@hostmaster-desktop:~$ su
Password:
root@hostmaster-desktop:/home/hostmaster#

The “#” indicate that you are login as root.