Issue sharing a PowerPoint in Teams Live on a Mac

Help needed! Can anybody tell me why I am getting this message on my Mac when I am trying to share content (a PowerPoint slide saved on my desktop) for a Teams Live event: 'Where’s your shared window? Sharing is paused until you return to the shared window'. I am using a single monitor but it won’t allow me to select the shared content window to send to the ‘Queue’ window. If I return to the shared window I lose the full screen of the PowerPoint. Why is this happening? Any help to resolve this appreciated as we have regular Teams Live sessions.