Simple Netlify sample app not working, API_ERROR

I am going through the steps on this page: A Complete CMS with No Server and 18 Lines of Code

It is supposed to create a very small app that can modify a Github repo. It says to create a totally blank repo and then use the two-file app to send some data into the repo. I am not certain how this is supposed to work but when I click on publish it sends an API_ERROR to the screen. This is what appears in the console log of the browser:

react_devtools_backend.js:4026 API_ERROR: Reference does not exist
at new t (https://unpkg.com/netlify-cms/dist/cms.js:54:164061)
at new i (https://unpkg.com/netlify-cms/dist/cms.js:54:164427)
at k.handleRequestError (https://unpkg.com/netlify-cms/dist/cms.js:54:241077)
at k.request (https://unpkg.com/netlify-cms/dist/cms.js:54:241572)
at async k.patchRef (https://unpkg.com/netlify-cms/dist/cms.js:58:5502)
at async t.runWithLock (https://unpkg.com/netlify-cms/dist/cms.js:228:13270)
at async Y.persistEntry (https://unpkg.com/netlify-cms/dist/cms.js:34:39529)
at async Object.onPersist (https://unpkg.com/netlify-cms/dist/cms.js:280:59862)

For the script source I have tried both “https://unpkg.com/netlify-cms/dist/cms.js” and “https://cdn.jsdelivr.net/npm/netlify-cms@^2.0.0/dist/netlify-cms.js”. Is there more complete documentation somewhere for this simple application to use Netlify?

Have you checked this: