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”
[email protected]:~$ sudo passwd
[sudo] password for hostmaster:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
To login as root, type “su”
[email protected]:~$ su
The “#” indicate that you are login as root.