https://www.youtube.com/playlist?list=PL3o94RqdqqlnwzbxvOP-chC3WEIEscT5f

The video playlist is privated. See screenshot here: https://imgur.com/a/NL6TlC3

If you have the link to the playlist and someone else privated it, you'll no longer be able to access it, even though you can still go to the URL where it was located.

If you know the owner of the playlist, you can consider contacting them to see if they can make it public and try again, but until then, downloading private videos is not something you can do unless you, yourself, made the video private.