Enable Root User on Ubuntu

1. Set password for root user:

Login as user
Open terminal
Issue the below command to set password for root user

$sudo passwd root

Give the password for root user 

2. Open the file 50-ubuntu.conf under the path (/usr/share/lightdm/lightdm.conf.d/)

Add the below line to it.

greeter-show-manual-login=true

Save and close the file.

3. Reboot the machine and now we can login as root user.