I have a tenant with a developer site collection in it. But it is not possible to upload an app with an .sppkg extension to the 'App for SharePoint' page. There is no "New" option, neither an Upload link, that everyone else seems to have right here. The very fact I am getting here tells me I have an App Catalog, but I have never got at the "Create an App Catalog" option. I am instead routed to a page that suggests how to get going with my App Catalog site. I have been told that it takes an "Developer Tenant" for this to work. What is the exact definition of a Developer Tenant? I just created a Developer site collection for a customer, exactly the way I did for my own stuff, and here the Create Catalog option comes up! In my own tenant (not for the customer), I also have other site collections in parallel with my Developer Site, of Team type. Is that what breaks it? Do I need to remove those Site Collections that are not of Developer type?