When you install the Debian (jessie) first time, root access for Open SSH server is not enabled for security reasons. If you want to enable the root's ssh access follow the instructions below.
SSH server's config is at
/etc/ssh/sshd_config. You must be root to edit file. Open the configuration file with your editor (e.g.
nano) and change the following line (most probably between line 20 and 30)
Once you edited the configuration file you should (re)start the SSH Server to make the changes effective. Use this command to restart the server as root.
systemctl restart ssh
Check ssh service status to see if everything is OK
systemctl status ssh