utbuntu 16.04 setup root first passowrd 
http://askubuntu.com/questions/44418/how-to-enable-root-login

Sequence worked:
tom@laptop:~$ sudo passwd -u root
[sudo] password for tom: 
passwd: unlocking the password would result in a passwordless account.
You should set a password with usermod -p to unlock the password of this account.
tom@laptop:~$ sudo passwd root
Enter new UNIX password: 
Retype new UNIX password: 
passwd: password updated successfully
tom@laptop:~$ su 
Password: 
root@laptop:/home/tom# 






