Most of the time Teams will not enable my webcam. Teams recognises that I have 2 cameras, you can see them in the drop down list. But the option to turn on the camera is constantly disabled.
However I also find that most of the time trying to start a video conference will just crash teams. Eventually it open the meeting but I get a banner telling me that "video is not working"
I've just reinstalled Microsoft Teams and everything runs ok, only to the point where I try to start a video chat. Prior to this I used a work issued device and my own personal phone and I'm able to connect with video without any issue.
Most common reason is that you have some other software running that is using your webcam or holding it so Teams can't use it. If you have Skype, Zoom, Snap Camera or something else running on your computer try to turn off that application and try again.