What does the message "No available sessions on the server" mean (v. 4)?

This article applies to NoMachine v. 4. Starting from v. 5, NoMachine is able to create an X11 display when no X servers are running on this host (e.g. headless machine) to let users connect to the desktop.

If, when connecting to a remote Linux host v. 4, you receive the following message 'No available sessions the server', it is likely that you don't have an X server running or there is no virtual desktop functionality available.

As explained in the article: https://www.nomachine.com/AR10K00702, the free NoMachine package v. 4 requires that the X server is up and running.

If your remote Linux server does not have an X server running there, you may install the Workstation.  Given that a desktop environment is installed on the remote machine, It will let you run a virtual Linux desktop there without the need to have the X server up and running.

 If a desktop environment is not installed but an X application like Xterm is present, the Workstation will let users to run such application as a virtual custom application.

Alternatively, you can still consider to use the free NoMachine package by following instructions here on how to connect to a headless server: https://www.nomachine.com/AR10K00710