Couple of things to check while you’re awaiting a response from dev/staff team. First, was the webinar deleted? It should still show up on your Past Webinars tab. If it’s been deleted, I don’t know if you can still get stats for it.
Second, for past webinars you have to find the webinar instance’s uuid and use that in the metrics call instead of the main webinar id that you’re using.
Also due note you must have at least 2 people in the webinar. After looking at your webinar ID, I see there was only 1 person, you, so it will not show participant info.