sharepoint online – How to fix problems downloading the application hosted by my provider

I've followed some instructions to the letter on a youtube video to deploy an add-on hosted by a provider hosted in Visual Studio 2017. The youtube video also matches the Microsoft documentation. I signed up for a new O365 hosted client and have put into use a collection of unique developer sites in the SharePoint Administration Center.

Still, at the last step where I download my .app file to the developer site, SharePoint provides a vague error message. How can I get a more detailed error message to try to figure out what's wrong?

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