Are you sure you have downloaded videos from khanacademy.org?Deriving from
these 2 FAQs:

<https://support.khanacademy.org/hc/en-us/articles/115001113931-What-should-I-do-if-YouTube-is-blocked-for-my-district>-

<https://support.khanacademy.org/hc/en-us/articles/202487500-Why-are-Khan-Academy-videos-not-playing-for-me>-

Khan Academy uses Youtube infrastructure to stream their videos.
And these are not possible to download with HTTrack.