Do you mean "root" does not appear in the list of users at the login screen, or does it appear as a user and the password doesn't work?
If the former, then for what it's worth, that's the way I prefer it. You should never have to log in as root unless in an emergency, when chances are you won't get to the GUI login anyway.
Go to "Configure your desktop", then to "Login Screen" under "System Administration", then go to the "Users" tab. You will see a list of "Excluded Users." Any user whose box is ticked, won't have his name appear at the login screen. I have my "root" ticked because I can't think of any reason to ever log in as root.
Is "root" ticked in your "Excluded Users" screen?