Additional features, settings, or issues not covered by specific Microsoft Teams categories
I experienced the same problem while teaching one of my courses online. The easiest fix is to download MacOS 11.0.1 Big Sur. This upgrade solved my problem.
If you are using an older MacOS open Preferences > Privacy and Security > Display > ensure MS Teams is checked. DO NOT CLOSE the preferences window, leave it open on your screen. I was able to screen share as usual.