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