Thanks to Rubén's comment, I looked at the execution transcript, and found this error:
[19-11-28 15:11:32:379 PST] Starting execution
[19-11-28 15:11:32:498 PST] SlidesApp.getActivePresentation() [0.114 seconds]
[19-11-28 15:11:32:570 PST] Execution failed: Authorization is required to perform that action. (line 6, file "progress") [0 seconds total runtime]
Publish a Google Add-on to the G Suite Marketplace
You must use the G Suite Marketplace SDK in order to get to the
“Publish” section for publishing an add-on, and you must first open
the Google Cloud Platform for your specific project (apps script
The G Suite Marketplace SDK must be enabled in the Google Cloud
The two ...
A Google Apps Script web app will have it's own URL to be used to make HTTP GET/POST requests.
A G Suite Add-on adds a menu to a G Suite app (Google Docs, Google Forms, Google Sheets, Google Slides)
A Google Apps Script project could be bounded or standalone and could be used to post a G Suite add-on, a web app or both.