If Mac users experience a screensharing issue, it is likely related to updated security settings in the new Catalina OS.

To utilize Webex screen sharing functionality during meetings, click OK after the prompt appears. If you click Don’t Allow on the desktop app, you can still grant permissions in your Mac device’s System Preferences.

Go to System Preference on your Mac > Security & Privacy.

Go to the Privacy tab, and in the Screen Recording section, enable Cisco Webex Meeting.

If you have already started the Webex event, you will likely need to log back out and restart the meeting for the updated settings to take effect.  

For additional details, see this article for instructions and screenshots:

https://help.webex.com/en-us/73zvl3/Support-for-Cisco-Webex-Meetings-for-macOS-Catalina#:~:text=If%20you%20click%20Don't,section%2C%20enable%20Cisco%20Webex%20Meeting