vi /etc/ssh/sshd_config or nano /etc/ssh/sshd_config
start from ubuntu 14.0.4 version root default login is couldn’t run SSH, you need to run below setting to configure it.
Run this command to open the SSH folder “ /etc/ssh/sshd_config “
Once you in the folder you will found this line “PermitRootLogin without-password“
Edit this line into “PermitRootLogin yes“ save it and exit.
Run this command “service ssh restart “ to restart the SSH service.
Run the putty and try to SSH login the server.