Sometimes other applications that use the camera and audio continue to persist to run and will not allow Google Meet to access the device's camera or microphone.
One such application is Microsoft Teams as it continues to run in the background when done using it. This app will keep Google Meet from accessing the audio or camera. You can close Teams by finding the icon in Windows 10 system tray, right click on the Team icon:
When you right click on it, choose Quit
Optionally, you can choose for it not to run after use in the Settings: