When NoMachine prompts you to insert the username and password, you need to insert the credentials of the account of the Windows machine. Remote login to Windows with empty password is forbidden by Windows policies: https://www.nomachine.com/AR05L00802.
But it is possible to connect without using system password. There are two ways to achieve it.
1. Connect with using private key authentication (https://www.nomachine.com/AR02L00785).
2. Enable NoMachine database. To do it – edit server.cfg as an administrator (in the default it’s located in the %PROGRAMFILES(x86)%\NoMachine\etc directory).Uncomment and change the keys to following values:
EnableUserDB 1
EnablePassDB 1
More details you can find here: https://www.nomachine.com/DT02O00125#10.1.