Replicas remains unassigned

[Hari_Prasad]

Hi I am currently in 6.4.2 version and I have lot of replica shards in "unassigned" state.

Below is the result That I get when I run query

GET /_cluster/allocation/explain
{
  "index": "log-2019.01.29",
  "shard": 0,
  "primary": false
}

"index": "log-2019.01.29",
  "shard": 0,
  "primary": false,
  "current_state": "unassigned",
  "unassigned_info": {
"reason": "ALLOCATION_FAILED",
"at": "2019-02-05T17:27:34.293Z",
"failed_allocation_attempts": 5,
"details": "failed shard on node [nusIaGG7SRWtv0Dw50KtiQ]: failed to update mapping for index, failure MapperParsingException[Failed to parse mapping [doc]: Root mapping definition has unsupported parameters:  [_size : {enabled=true}]]; nested: MapperParsingException[Root mapping definition has unsupported parameters:  [_size : {enabled=true}]]; ",
"last_allocation_status": "no_attempt"
  },
  "can_allocate": "no",
  "allocate_explanation": "cannot allocate because allocation is not permitted to any of the nodes"

I have already tried the POST /_cluster/reroute?retry_failed but still the same result

Below is the template that I have in logstash

{
    "template": "log",
    "index_patterns": ["log*"],
    "settings" : {
        "number_of_shards" : 5
    },
    "mappings":{
      "doc":{
         "_size": {
            "enabled": true
         }
      }
    }
}

PS: I have mapper size plugin enabled on all the nodes and this exception on only replicas seems confusing

Thank you

[Hari_Prasad]

This was my caused by my mistake. The mapper size plugin was not installed on all the Elasticsearch nodes.
Once this was corrected the replicas started getting assigned.