Why does a desktop show as unavailable?

A desktop shows as unavailable when the display that it belongs to isn't currently connected, the desktop was removed in Mission Control, or ScreenPlace is unable to access your desktops. A desktop of a display that isn't currently connected will become available again once you connect it. You can remove an unavailable desktop on the screen for editing a workspace by using the Remove button next to it. Please just note that desktops are specific to your Mac, meaning that the desktops in a backup that you restore on another Mac might show as unavailable, in which case you will need to select them again.