The account root
is already there, as other have pointed out. It's a very special one, not your typical user account, so it's usually hidden from most graphical tools out there.
You're not supposed to use that account directly, but through sudo
only when neccessary. By default, the login with that account is disabled, and it should stay that way for most people.
If you know what you're doing, You can still enable that account with this command:
sudo passwd
That will let you assign a password to root and enable its login. This has an only advantage for typical installations AFAIK: automatically enable password-protected recovery mode, otherwise completely unprotected.
Even when enabled, the root
account will still be hidden from most graphical tools. Some can be configured to show it, some don't.