If you are using MacOS Catalina, you must update the privacy settings on your Mac in order to grant Screen recording permission to your web browser.
Open System Preferences and select Security & Privacy.
Click the lock icon in the bottom left and enter your credentials to allow make changes on this section.
Under Security & Privacy click Privacy, Screen Recording and check the box of the web browser that you want to grand screen sharing permission.
You may need to close your web browser and reopen it before the changes take effect.