By default due to security reason you are not allowed to use root account with password to login to Debian 8
Assume that your ssh is already installed navigate to /etc/ssh/sshd_config and amend the following configuration
PermitRootLogin without-password to
Once this is done you need to reload your ssh service
Once this is completed you will be able to login using ssh.
This is a good practices as long as you are using it for your virtual dev environment. Due to security concert never using in production server.