I’m afraid the reason is that you are trying to connect by ssh to a free NoMachine install. The free version doesn’t support SSH. Be sure you select the NX protocol in the client settings.
There are good reasons why SSH is not in the free version. People expect software to install and work out of the box, while SSH requires that you install and configure SSH beforehand. This is not what the average user expects (in this respect previous NoMachine versions were the exception, rather than the rule). Second SSH can be tricky to configure. If you search the Internet you easily realize that 90% of problems people had with versions before the 4 were with configuring SSH, rather than the software itself. Probably NoMachine thinks support is better employed at handling the paying customers, rather than the users of the free version. Third the NX protocol can use both TCP and UDP for multimedia traffic and this can make a huge performance difference. Clearly NoMachine wants people to get the best from the software. If you are in a setting where you can’t open the port on the router, or are smart enough to know how to configure SSH, you can still tunnel the NX connection over SSH, as you would do for any other protocol.