After fresh system installation the root login on the Debian Linux is disabled by default. When you attempt to login as root user to your Debian Jessie Linux server the access will be denied eg.:
$ ssh This email address is being protected from spambots. You need JavaScript enabled to view it.
This email address is being protected from spambots. You need JavaScript enabled to view it.'s password: 
Permission denied, please try again.
This email address is being protected from spambots. You need JavaScript enabled to view it.'s password: 
Permission denied, please try again.
This email address is being protected from spambots. You need JavaScript enabled to view it.'s password: 
Permission denied (publickey,password).
To enable SSH login for a root user on Debian Linux system you need to first configure SSH server. Open /etc/ssh/sshd_config and change the following line:
FROM:
PermitRootLogin without-password
TO:
PermitRootLogin yes
Once you made the above change restart your SSH server:
# /etc/init.d/ssh restart
[ ok ] Restarting ssh (via systemctl): ssh.service.
From now on you will be able to ssh login as a root:
$ ssh This email address is being protected from spambots. You need JavaScript enabled to view it.
This email address is being protected from spambots. You need JavaScript enabled to view it.'s password: 

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
The following guide will provide you with the information on how to enable SSH root login on Ubuntu 16.04 Xenial Xerus Linux Server/Desktop.