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
Set root password:
sudo passwd root