to the best of my knowledge, that's because your system does not have ffmpeg for subsequent transcoding of the site's format into what you would consider "your" format: https://github.com/yt-dlp/yt-dlp/blob/2023.07.06/README.md#s... (they also allow you to specify the path, if you don't have it in your $PATH, via --ffmpeg-location <https://github.com/yt-dlp/yt-dlp/blob/2023.07.06/README.md?p...>)