In most cases where screen sharing doesn’t work, it’s because the screen sharing function is blocked by your browser. This can easily be fixed via your browser settings (found near your search bar).
It might also be that your operating system (macOS or Windows) has removed permission for your browser to access screen sharing mode. To change this, you have to open system preferences and allow the browser you’re using to access screen sharing.
Tip: If you’re in a hurry, you can also just switch browsers.