How to understand the error of reindex (request body is required)?

Elastic Stack Elasticsearch
5

So, judging from the error message and the stack trace it seems, that there is a HTTP body missing. Your sample above looks good

POST _reindex?slices=5&refresh
{
  "conflicts": "proceed",
  "source": {
    "index": "iotmessages-000001"
  },
  "dest": {
    "index": "iot_prd_iotmessage-000001"
  }
}

Just to make sure this is a not a possible error. Did you remove any newlines between the first POST and following lines, so this is treated properly by the Kibana console?

If that does not work for whatever reasons, can you try running this as a request using curl?