If you’re not able to share your screen on MacOS, it is likely that the screen recording feature needs to be enabled for your browser within the device's settings.
On newer operating systems:
Open System Settings. Navigate to Privacy & Security > Screen Recording. Check the box next to the browser you are using. Restart your browser for the changes to go into effect.
On older operating systems:
Open System Preferences. Navigate to Security & Privacy > Privacy > Screen Recording. Check the box next to the browser you are using. Restart your browser for the changes to go into effect.