The error message you are getting says that openCV can't load the H264 codec. H264 doesn't come by default with the default installation of openCV.

To add the H264 codec download 'openh264-1.6.0-win64msvc.dll.bz2' from https://github.com/cisco/openh264/releases/tag/v1.6.0.

Extract the file and move the extracted DLL to the same directory as your python file. OpenCV should now be able to find the DLL and load the H264 codec.