The root user in Xubuntu has restrictions for security reasons. You should use the sudo command instead.
I wanted to login as root and use “su root” but got authentication failure errors in the login screen and in the terminal.
I had issue “sudo passwd root” from the terminal to set up the root user (it will prompt to create the root password although it asks you that during the installation…) which fix the authentication errors.
I also changed the “login window” settings to allow admin login. Now I am able to login as root!
