1) First
enable the root account by running the commands below:
$ sudo passwd -u root
When prompted for password, enter your personal password to continue.
2) Then
reset/add the root password by running the commands below:
$sudo passwd root
When you run the commands above, you'll get prompted to enter a new password.
reference link: Liberian Geek
[Question] What is the Root Default Password in Ubuntu 12.04?