When an error message is shown from a python code using cv2 (opencv), the message is most likely misleading. The message is saying nothing but something is wrong. All the details in the message are not what you need to do to fix the problem.
I had issues with VideoCapture and imshow. The error messages look explanatory but they are not. All problems were stemmed from a python wrapper package that I installed through pip.
$pip uninstall opencv-python
Your conda must have one opencv version. In my case, the version is 3.2.0 that was built from source code where I enabled FFMPEG to fix VideoCapture problem.