Does Free Developer Account Work?

I've seen several posts on this community about oAuth not working on the Adobe Sign Rest API, for the Free Developer Account, as outlined in the "Getting Started" instructions.

https://www.adobe.io/apis/documentcloud/sign/docs.html#!adobedocs/adobe-sign/master/gstarted.md

I have this problem and I have not seen a single solution. But there have been posts from Adobe employees stating that integration keys don't work on single user license, that an Enterprise account is needed.  I don't see how "integration key" or "Enterprise Account" are relevant to the issue. Is there something I'm missing?

Does anyone here know of someone with a Free Adobe Sign Developer Account being able to use the oAuth process? And if so, how did they do it?