If you get an error showing Camera is not available when selecting your camera device, it is likely that your camera is currently in use by another application.
The error will also indicate which process is currently using your camera device.
For streaming software, remove your webcam source from all of your scenes.
For video chat software, check if it's running with your camera device in your video settings or by closing the video chat app. Most video chat apps run in the background and you can check your Windows system tray or your task manager to close it.