Hellp
 

Why can't you use the command "sudo su root" from a user besides Ubuntu?

Not entirely sure I understand your question but I will give it a shot.

If your user is having an issue using sudo, ensure you are in the sudoers file.
If you are able to use sudo for other commands, just not 'su root', check the sudoers file for which commands the user has access to via sudo.

What I have read it that, when Ubuntu is installed, it puts the first user (usually the installing user) in the sudoers file. Other users are not automatically added to the sudoers file.

See man visudo for information on how to edit the sudoers file in Ubuntu. I admit, it's kind of murky, but I've never done it myself.