If you are using Video Sensing to make interactive game in Scratch 3.0, you might meet the issue that the camera seems not being able to be turned on. Recently, I met this issue. I searched Scratch discussion forum and noticed someone had reported the same problem but no answer was given in the post, but luckily, I found what caused this issue. It might be due to the setting of your browser.
Webcam is not allowed
Using Chrome
If you are using Chrome, once adding Video Sensing module from Extension Tab, you could see a video icon at the right side of the address bar, as shown below. If the video icon shows differently, it means the webcam is not allowed on this webpage, yet.

Click this video icon and a dialog will pop up as shown below. Check option “Continue allowing https://scratch.mit.edu to access your camera”.

Click “Manage” button to open Chrome Setting dialog. Ensure to turn on “Ask before accessing (recommended)” option and put “https://scratch.mit.edu” into the Allow list, as shown below.

Now your could close Chrome Setting dialog. Refresh your Scratch project and the video icon would show normally.
Using Microsoft Edge
If you are using Microsoft Edge to open Scratch website, this message will pop up:

Just click “Allow” and your Scratch project could use the camera.
However, if you did not see the above dialog, you could check the address bar just like what I did in Chrome.

When Other Applications Are Using Camera
If other applications are using camera, Scratch project could not access it at the same time. For example, you might have a Zoom session with others to demonstrate this video sensing project. Since the camera has been used by Zoom, Scratch project could not use it. What you should do is to turn off video in Zoom first. The camera might not be able to work immediately. You need to refresh your Scratch project and the camera could be turned on after a few seconds.
Hope this tip is helpful. Enjoy the coding and have fun!
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