MacOS requires the user to allow applications to access the camera. If access is not granted by the user the operating system will not allow access to the camera.

If the camera is not active, you have likely not given the app permission when the dialog for camera access asks for this permission.

In order to grant the app access to the camera you must go to the System Preferences --> Security & Privacy menu and select the Camera category in the Privacy tab:

In order to make changes you may need to click on the padlock at the bottom left of the dialog and enter your password.

If the check box for is already checked, then you need to uncheck it and check it again. If it is not checked, simply check the box and then close the window.