Hello,
using the same steps in the tutorial, we tried to replicate your problem and were able to connect to an AWS EC2 instance (Ubuntu 16.04).
After installation of NoMachine on Ubuntu, I did have to create the .nx directory since it was not created automatically. So, after you connect to your instance via ssh and install NoMachine, you might need to do the following:
– create .nx directory manually (mkdir .nx)
– create config directory inside .nx (mkdir config)
– create file authorized.crt (touch authorized.crt)
– view content of directory .ssh/authorized_keys and append it to .nx/config/authorized.crt (cat ~/.ssh/authorized_keys >> ~/.nx/config/authorized.crt)
After you do this and follow the rest of instructions from tutorial you should be okay and able to connect. Let us know if it worked for you. Good luck.
Greetings