If you are using Safari to share your screen, and you accidentally disallowed it in the past when asked, please click Safari at the top left > then click Settings for this Website
Make sure to set Camera/Microphone/Screen Sharing to Allow or Ask
Google Chrome and Microsoft Edge on Mac OS
If Mac OS asks you, click Open System Preferences
When your Mac OS asks for your permission and you denied it, please open your System Preferences.
Inside System Preferences, click Security & Privacy
Click on the Privacy tab
Click the lock found at the lower left of the screen to enable it
Look for Screen Recording
Enable Google Chrome/Microsoft Edge
While you are here, make sure Camera and Microphone are also enabled for the browser you will be using.
Close System Preferences and try using Google Chrome or Edge. Screen sharing should work now.