This is a harmless error message. You can get rid of this by adding a second argument cv2.CAP_GSTREAMER after your pipeline string in cv2.videoCapture.
cv2.VideoCapture("nvarguscamerasrc ... appsink", cv2.CAP_GSTREAMER)
When no mode is specified, opencv may try to interpret the provided string for various kind of sources: V4L device, file if the string ends with an extension (has a dot) which is not your case, or gstreamer pipeline.
When it tries as V4L device, if you have built your opencv version with libv4l/libv4l2 support, then this message is issued.