A lot of newer installs will automatically prohibit the root user from logging in directly, for security reasons or they will only allow key based access.
If you know what you are doing/don't care about security or have an incredibly secure password for testing, then you can enable it.
Edit this file: /etc/ssh/sshd_config
Find the following line: PermitRootLogin
Set it like this:
In Debian a lot of times SSH disables the root user to login by password by default. This means you will get an authentication failure as if you typed in the wrong password.
The logs also indicate the password is wrong but what is often the case is in the config file
cat /etc/ssh/sshd_config|grep -i permitrootlogin
Make sure it says:
If not change it and restart SSH........