Several configurations not allow root login. Follow these steps to allow root login.

SSH Configuration Update

In order to allow root login, you have to edit /etc/ssh/sshd_config file:

sudo nano /etc/ssh/sshd_config

Then, change the PermitRootLogin value from without-password to yes. Save and exit.

Restart SSH service with this command:

sudo service ssh restart

Root Password

Set root password:

sudo passwd root