The fhir-cda project (https://github.com/hl7/fhir-cda) contains 7 locations where a healthintersections extension is used. I would expect a FHIR core extension instead. From the qa.html:

The extension http://www.healthintersections.com.au/fhir/StructureDefinition/elementdefinition-dateformat is unknown, and not allowed here

The thing with this error is that you get this after building the |IG. This extension does not live in the source as far as I could find, so I'm guessing it is added during publication.