ffmpeg error

while i was trying to run this code, i'm getting following error

SystemError: <class 'cv2.VideoWriter'> returned a result with an error set
warning: Error opening file (/build/opencv/modules/videoio/src/cap_ffmpeg_impl.hpp:834)
warning: C (/build/opencv/modules/videoio/src/cap_ffmpeg_impl.hpp:835)

i've installed ffmpeg library , also added to path. still i'm unable to resolve