If a Host or Presenter within your event is reporting that they are unable to share their screen - it may be that they have not allowed screen recording on their web browser. Just note, screen-share permissions is a Mac/iOS issue only.
Below are instructions on how to troubleshoot this issue on a Mac:
Open up System Preferences on your Apple device
Select Privacy & Security
Scroll down until you find Screen Recording and select it
Your web browser should be checked. If it is not checked, select the lock on the bottom left-hand side of your window to make changes - this will require you to enter your password.
Check the box next to your web browser and select the lock again to save
Refresh your web browser (sometimes it may require a full quit out of Chrome) and that's it!
Updates to Chrome may require you to repeat the steps above to allow for screen-sharing in the future - be sure to check your screen-share permissions periodically to ensure you do not run into any errors!