error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor

[datasciencecompleteguide]

Check the above screen shot.
In this line 3 where i am capturing video, I have passed a Integer number 3 and I got the same error as described by you.

I have three webcams with me and whenever i use them in OpenCV, I access them with Three Integers (0, 1 and 2).

But when i have passed a unknown or non-exsisting source (i.e., 3)and tried to convert the color of the frame in "line 8" then it shows the error src is empty.

Means i was unable to read the image properly in line 3.

Thus, you also need to check the source.
Project: Face Mask Detection

If further you are getting error then pls share the piece of code.