If the screen sharing feature is not available in MS Teams on macOS there is a chance that the feature is being disabled in the Security & Privacy. Please follow the below steps to enable the settings.

1- Go to the Launchpad & select System Preferences

2- In System Preferences, select Security & Privacy 

3- In the Security & Privacy, go to the Privacy tab & select Screen Recording. Make sure Microsoft Teams are selected. (Click on the lock before making any changes)

4- While you are in the Privacy tab, check the Camera & Microphone just in case they are not enabled.