Some time last year, Youtube stopped serving up usable videos in any resolution higher than 720p. They used to offer 1080p and 4K (formats 37 and 38) but those are no longer offered. Apparently the new party line is that for anything higher than 720p, you are expected to download a 1080p video stream that has no audio in it, and a separate audio stream that has no video in it, and mux them on the client side.
Which is convenient and awesome.
So now youtubedown does that, if you have ffmpeg installed. If you don't have ffmpeg, or if you specify