Check if spacedeskService is operating. 
Make sure that spacedesk service has started and spacedesk System Tray icon (at the bottom right of the screen) is visible in taskbar’s notification area and turned ON.

spacedesk server can be enabled and disabled via spacedesk Driver Console app. 
In case viewer app cannot discover / connect to the server, make sure that the spacedesk Driver toggle switch is ON and Local Area Network box is checked in Communication Interfaces.

Check the above chapter Verify if Setup was Successful for details.