Download media from a plethora of sources, even when embedded in other pages.
The main commands are dv to download video and da to download audio (often extracted from a video source). Both will present you with two download options, if available: the URL of your frontmost browser tab, and the URL in your clipboard.
Run any option with the ⌘ modifier, and the full playlist will be downloaded. Use ⌥ and it will toggle adding to WatchList (a separate Workflow).
What was the output of the command? If you run it now, what does it say?
And only in that situation?
Open a terminal and run youtube-dl --update, in case you have it installed from another source.
If that doesn’t work, open the debugger, set the log output to “All Information”, and post the output.