Cannot login on SSH

I have Linux running on a virtual machine, trying to log in from the host OS:

$ ssh -l root 172.16.33.1
root@172.16.33.1's password:
Permission denied, please try again.
root@172.16.33.1's password:
Permission denied, please try again.
root@172.16.33.1's password:
Permission denied (publickey,password).

Inside the VM:

# cat /etc/ssh/sshd_config
PermitRootLogin yes
PasswordAuthentication yes
UseLogin yes

What else is missing?