You need to make sure SIM EFB manager is running at the same time as FS2020. If opened later you may see this. If you hover the mouse over the "S" of "SIM EFB 1" a refresh button will appear. Press it and it will reload. ONLY of the manger is running at the same time.
Bobaz, Did you ever get this problem resolved? If not, are you running multiple monitors or do you have USB devices such as Oculus Quest 2 connected when installing the program. What happens when you try to publish "External Panel"?