Possible Causes and Responses
Audio-only video
Audio-only files that do not contain any images (pictures, videos, animations) cannot be registered.
Please add video or prepare a video with images.
Related: Are there restrictions to the total amount and format of image/video data that I can upload?
https://help.teachme.jp/hc/en-us/articles/900006441863
Example of how to add pictures to audio
Convert with the standard Windows "Photo" app
Import the audio into the "Photo" application, add title cards, images, etc., and export as a video with the "mp4" extension.
If you are still experiencing troubles
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