"Unsupported URL: https://www.youtube.com/"

How to grab a youtube video if youtube-dl reports "ERROR: Unsupported URL: https://www.youtube.com/ "? Complete error: [generic] watch?app=desktop: Requesting header [redirect] Following redirect to https://www.youtube.com/ [generic] www.youtube: Requesting header WARNING: Falling back on generic information extractor. [generic] www.youtube: Downloading webpage [generic] www.youtube: Extracting information ERROR: Unsupported URL: https://www.youtube.com/