At this moment NordVPN Linux application does not have any graphical user interface and the application is used by entering the commands via the Linux terminal window.
Why do I see a NordVPN icon in the Taskbar Tray/Applications list? This icon is automatically generated by Linux whenever a successful installation of NordVPN has been established. Nevertheless, clicking it will not lead to a GUI.
In order to start using NordVPN on Linux, please follow this guide.