The behavior you see

You click the screenshare button and select a screen to share, but your screen doesn't get added

What's causing the problem

Most likely your system is blocking your browser from sharing your screen

The solution

Give your browser permission to share your screen. For example, on a Mac:

1. System Preferences
2. Privacy & Security
3. Privacy
4. Screen Recording
5. Ensure your browser (Chrome) is Allowed

Try restarting your browser