Sharing window or application is fine but sharing whole screen is always black. I’ve tried this on google chrome, microsoft edge. I have attached a screenshot.
It might be a problem with Wayland. Can you try sharing your screen with Xorg? You can change from Wayland to Xorg on the login screen (bottom right).
There was no Xorg option but I went with Plasma X11 and it worked. Shouldn’t be Wayland more advanced than X11?
I’m not a specialist, but Wayland should be the more modern option, but there can be some bugs or problems with certain types of hardware. If memory serves me right: it works better with AMD GPU’s than with Nvidia.