To enable screen sharing on Mac (OS Catalina) your guest will be required to enable a setting directly in System Preferences > Security & Privacy > Privacy.
After clicking the padlock on the bottom-left to make changes, scroll to Screen Recording settings. Enable the checkbox next to the preferred browser to allow the browser to access screen recording.
If after enabling this option screen sharing is still not working. Navigate to System Preferences > Sharing.
Select the Screen Sharing option, then allow access for the user signed in.