When I using red hat or centos, no problemo at all with root login. It is the first time installing Ubuntu, for the first time you must create user like windows xp. The user is administrator but not superuser like root. To get into root login you must set root password first, using user that created for the first time.
sudo passwd root
Enter new UNIX password:
Retype new UNIX password:
password updated successfully
And then you can login using root with new password