How to enable root login on ubuntu 16.04 ?

Enable root logins first you have to set a password for the root account and then unlock the locked root account. If you don’t set a password for the root account the passwd command will return
passwd: unlocking the password would result in a passwordless account.  So, first execute in a terminal

sudo passwd root

you will be prompted for a new Unix password. Write it twice (second for confirmation).
To unlock the account.

sudo passwd -u root

passwd: password expiry information changed
Reverting back
If you want to disable root account in Ubuntu you need to lock the root account by using the following command

sudo passwd -l root

Alternatives to root login
If you want to work on a root console you can also use

sudo -i