If you are using a Mac computer and try to share your screen for the first time you will be prompted to enable screen recording in your Mac Privacy and Security Settings. 

• To open this pane on your Mac, choose Apple menu  > System Preferences, click Security & Privacy, then click Privacy.

• Ensure that Google Chrome and/or the Desktop App you are running is selected and these settings are saved. See the screenshot below.

• You will need to quit the app or browser and rejoin the meeting for the permission to take effect.