Error type: "Not Found". Error message: "The playlist identified with the requests playlistId parameter cannot be found." Domain: "youtube.playlistItem". Reason: "playlistNotFound". Location type: "parameter". Location: "playlistId".

Did you added your own Google API key? Look at the help.

Check in YouTube if the id PLNgn21fIijB-8TCa7bhmqO69SN7zeDzK- belongs to a playlist. Check the FAQ of the plugin or send error messages to support.