Enabling
To actually 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).

Then execute

sudo passwd -u root 

to unlock the account. This should return

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

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