The product you downloaded is the free NoMachine for Linux which gives access to the physical display. If you want to use it on a headless server, just follow the instructions here:
https://www.nomachine.com/AR10K00710
If you want to connect to a virtual desktop (like you did with version 3, assuming you were using the free version back then which was the NX Free Edition), then you need the Workstation as I already mentioned.
I don’t think you had problems because the software didn’t work (it works out of the box) , it’s because you downloaded the wrong product.