If you got the message like this:

$ su - root
Password:
su: Authentication failure
Sorry.

Then you can input the command:
sudo passwd root
open the root user and input the passwd:

$ sudo passwd root
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

so that you can su -root

Want to close the root user in the ubuntu, run the command:
sudo passwd root -l