You need to edit /etc/ssh/sshd_config, and comment out the following line:
PermitRootLogin without-password
StrictModes yes
Just below it, add the following line:
PermitRootLogin yes
PasswordAuthentication yes
Then restart SSH:
service ssh reload
https://askubuntu.com/questions/469143/how-to-enable-ssh-root-access-on-ubuntu-14-04