error while opening camera

Hello,

Thanks for the tutorial.
I am facing an issue with the following code snippet. 
cap = cv2.VideoCapture(0)
And the error is

[ WARN:0] global /tmp/pip-req-build-1syr35c1/opencv/modules/videoio/src/cap_v4l.cpp (893) open VIDEOIO(V4L2:/dev/video0): can't open camera by index

Can you suggest any idea how I can make this `work?`

