Cannot currently show the desktop


I have loaded a few sd cards with the KM4ACK pi build and some with the W3DJS image. I set them all up using an ethernet connection and an hdmi connection. All goes well. I VNC into them and I can see the desktop on my computer or tablet. I shut down with no problems. When I disconnect the hdmi as well as the ethernet and try to just connect through VNC, I see the message "cannot currently show the desktop". What would be the display resolution that I need? I have tried a few different resolutions and when hdmi is plugged in my tablet sees the desktop. Without hdmi plugged in, no desktop. I know VNC is connecting to the pi but not able to display the desktop. Is there a different way to set screen resolutions other than from the configuration menu?
Any suggestions, please???

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