Correct! Registry tokens do not have authorization to the POST /submissions endpoint. This is because the entity you are serving receives measurementSets posted automatically from other CMS internal systems and POST /submissions could overwrite those and negatively affect your client. Only Individuals or Groups can use that endpoint. We are working to update the swaggar docs with a note to remind Registry users of this.
If you are a Registry the best endpoint to use is POST /measurementSets. When you POST the first Measurement Set for an entity it will also create the SubmissionsID for the entity you are posting for. Additionally you could use POST /measures.
Hope that helps!
Michelle