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 Driver Console window -> Connections Active (connected) page and on Adapter Interface -> current Network Adapter -> Active Connections.
Open Display Control Panel via spacedesk Tray icon, right-click -> Windows Display Control Panel menu then check if there’s a secondary display attached.