Warnning: this method might be unsafe for server users.

ubuntu don`t allow root user and also login in ssh as root by default.

so the first thing you have to do is to enable your root user account.

$ sudo passwd root

ok, you have got your root account enabled now.

I am not sure whether you have installed your OpenSSH server or not.

so I will teach you How to install OpenSSH server.

$ sudo apt-get install openssh-server

now you are able to edit the ssh config

$ sudo vi /etc/ssh/sshd_config

find the line contains « PermitRootLogin no » modify it to « PermitRootLogin yes »

oh yeah, now restart the ssh server.

$ sudo service ssh restart

now give it a try