Google Workspace, "Invalid Client ID" when adding Domain-wide delegation

I'm trying to setup a Service account to use the Google Calendar API on a website.

I'm mostly following this tutorial I'm Super Admin on the Google Workspace Admin Console.

I created the service account as explained on the tutorial, but when I try to add a new Domain-wide delegation, I get the "Invalid Client ID" error. I can't understand what's wrong.

Something weird I noticed is that on the Google Support page about the Domain-wide delegation, it says that I'm not Super Admin.

But on the Google Workspace admin console, I clearly am.

I don't know if this could be related to my problem