Why are there some aerobics videos that can be played on the computer but not when downloaded into a video player?

Maybe there is an error in the video format.

Windows comes with a video player that supports too few formats. Since this square dance video is not compatible with the system inside your phone. That's why it will cause the wrong format and then it can't be played.

You can solve the problem at its root by converting the downloaded video format to the common MP4 format. Or change the video player.