I want to enable the 'root' user during the login prompt on Ubuntu 14.04 server
Solution 1:
You should set password of root user to enable it. Use this:
sudo passwd root
Solution 2:
In /etc/ssh/sshd_config you need to comment or delete the line PermitRootLogin without-password
After that, add the line PermitRootLogin yes
Finally restart the SSH with service ssh restart