If the screen share button does not work, this is in most cases because you have previously not given your browser permission to do this. Access your system preferences and allow the browser you are using to share screen:
On Mac: System Preferences > Security & Privacy > Privacy > Screen Recording and add in Chrome.
On Windows: If screen sharing isn’t working this might be due to a missing ‘screen sharing extension’ in your browser. You can check this by typing chrome://extensions/ into your browser's address bar. In case the screen sharing extension is already in place it might be disabled, then switch it on. If the screen-sharing extension isn’t there, please visit the Chrome Web Store to install one for free via this link.
If you are in a hurry, try another browser. The platform is compatible with the latest Google Chrome and Microsoft Edge versions.