It can be that you are not able to share your screen on your Mac as this capability is by default disabled in latest versions. To be able to share your screen during the session, you need to grant permission for screen recording for Google Chrome:
- Go to System Preferences > Security & Privacy > Privacy.
- On the left, select Screen Recording.
- Check the box next to Google Chrome.
- Restart your browser for the changes to take effect.
You may have to click the lock icon at the bottom left and enter your Admin password in order to be able to make the above changes.