Verify Network Connection and additional Display Monitor

On the Primary Machine side, check if the connection is established. Just click the spacedesk System Tray icon (at the bottom right of the screen) on the primary machine to display the spacedesk SERVER user interface, then the number of successfully established connections can be seen and the attached device/machine in the Network Connections (remote) tab.

Open Display Control Panel via spacedesk SERVER user interface menu button then click Display Settings …, then check if there’s a secondary display attached.