If you are getting this error after you have chosen "Password authentication", password authentication is probably disabled on the remote server. To enable it, set the PasswordAuthentication yes option in the /etc/ssh/sshd_config file and restart the ssh server. i.e service ssh restart.

NoMachine doesn't limit in any way the accounts that can have access to the remote server (although connecting as root or with an administrative account is strongly discouraged, as explained in this article: https://www.nomachine.com/AR12K00763).