Root access is disabled

Securing and Optimizing Linux
Some SSH servers can disable SSH logins for certain users and may even disable root logins by default.
If you’re using Open SSH server, you have to edit /etc/ssh/sshd_config and make sure that it doesn’t contain one of the following lines:

PermitRootLogin no

PermitRootLogin without-password

Both result in a simple Access denied. error, although they permit you to enter the password. The latter only allows root logins when authenticating using public keys.
To permit root logins, remove the corresponding line or replace it with the following:

PermitRootLogin yes

Restart ssh server to see the effects.

sudo restart ssh

See man sshd_config for further information.