The preview shows the since multiple documents can be streamed, each document being streamed is a map. The REST POST snap will take each document and write it out to the endpoint, as a map. You can change the POST endpoint to a test endpoint created in a service like https://requestb.in/, to verify whether the right data is being posted.
The Zendesk API error response would have more details. It is possible the right content type was not set, add a HTTP Header in the POST snap, with key “Content-Type” and value “application/json”