Please see the following posts:
https://www.nomachine.com/forums/topic/no-available-sessions-on-this-server-error
https://www.nomachine.com/forums/topic/no-available-sessions-on-this-server-on-rhel
https://www.nomachine.com/forums/topic/no-session-on-headless-server
There is also the article here: https://www.nomachine.com/AR10L00818