I've recently upgraded to using Ubuntu Server 17.04 and I have noticed that by default you must create a user instead of being able to login as root, which is a great idea for security, but makes things harder for permissions, etc.
Once the Ubuntu 17 server has been set up, all we need to do is modify /etc/ssh/sshd_config and add the line PermitRootLogin yes underneath the Authentication heading.
sudo nano /etc/ssh/sshd_config
#Authentication: #LoginGraceTime 2m PermitRootLogin yes #StrictModes yes #MaxAuthTries 6 #MaxSessions 10
Once that's done, we just need to restart the ssh server
sudo systemctl restart sshd
Now, we need to make sure the root user has a password, if not, we can set one now.
Enter the new root password, once that's done, try logging in as root with SSH, it should work now!