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:
Now restart sshd
systemctl restart sshd