This usually means that the client side cannot reach the NoMachine service on the remote host. This can be for several reasons, but there are a few things you can try before asking for help on the forums or contacting NoMachine support.
 

1. Check that you have set the correct IP and Port

You can do this via the NoMachine User Interface by opening Server Preferences and the Services tab on the host you want to access, see also:

for v. 6 https://www.nomachine.com/DT10O00155#2

for v. 5 https://www.nomachine.com/DT11M00107#2

2. Check that the firewall configurations on both client and server side aren’t interfering

Generally, the firewall shouldn't be a problem if it’s native to the OS you are using. Refer to article:

https://www.nomachine.com/AR11L00827

3. Check nxserver status on the server side.

To check its status, open up the Server Preferences like you did for point 1) and make sure status is ‘Running’.

If you still can't connect, then the logs from both sides will need to be checked.