By default Ubuntu 18.04 installation comes with unset root password. To set root password open up terminal and execute the following linux command:
$ sudo passwd [sudo] password for user: Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully
Edit SSH configuration file: /etc/ssh/sshd_config and set property PermitRootLogin to yes:
Save configuration file and restart ssh service by command:
sudo service ssh restart
To restart SSH on Debian you can use command:
systemctl restart ssh.service