I think maybe it is because of the copyright reason. Sometimes even when I use an online YouTube downloader, an error message would occur saying that my video doesn't exist or is not allowed tobe converted. Maybe you can use a screen recorder to overcome this issue. Win + G for Windows 10, Quicktime for Mac. I myself use Joyoshare Screen Recorder (https://www.joyoshare.com/screen-recorder-for-win/) as an alternative. Recording them in 1080p at 60fps and saving in different formats would work as well.