1. Ubuntu disables the root user account by default. To enable it, open up a console and type sudo and then your regular user's password. Then type passwd root to set the root user's password. You should now be able to log in as root.
2. To set the root password use this command
It will first prompt for a password, this is your user's password. Then it will ask you the password to set for root. After you set the password, you can then login as root or su to root.
enable the root, then try again.
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