There are 3 things you can check:
1) that the NoMachine server is running on the Windows 11 host that you are trying to connect to:
open the NoMachine player > Settings > Server > Status. Does the server show as running? Are any IP addresses listed there. You can see an the example here: https://knowledgebase.nomachine.com/DT11R00177#3.1. If it isn’t, enable the server. (Btw, when you initially installed NoMachine did you restart the computer?)
2) Connecting to the localhost is an simple test to check that NoMachine is up and running correctly as well. On the Windows 11 computer, open the Player > Add connection > write ‘localhost’ in the field ‘Host’ and click ‘Connect’. A successful connection to localhost will show your desktop infinitely mirrored within the Player window.
3) If you’re connecting to the NoMachine server from a public network, verify that its public IP is visible in the list (point 1) and that your connection uses the port mapped on the router.
Some more useful tips are available here:
https://knowledgebase.nomachine.com/AR10R01099