YouTube constantly updates its coding architecture, algorithms, and streaming protocols (like DASH). A version of software from the era of 2.1.6 may effectively be today. Even if you successfully install it, the software will likely fail to fetch video URLs or crash when attempting to convert files because it was built for a version of YouTube that no longer exists.
If it fails, try using youtube.com/watch?v=XXXXX (not the shortened youtu.be link) and disable HTTPS if the app has such an option (though this is highly insecure). youtube downloader old version 2.1 6 free download
But I had a secret weapon.
Modern software frequently interrupts workflows with "update available" pop-ups. Version 2.1.6 was released before aggressive auto-update features became standard. YouTube constantly updates its coding architecture