How to fix cluster health yellow with Elastic Search

I have setup on server, with MongoDb and ElasticSearch. Using https://github.com/richardwilly98/elasticsearch-river-mongodb I have connected ElasticSearch and MongoDb together.

I create a new index using:

curl -XPUT 'http://127.0.0.1:9200/_river/mongodb/_meta' -d '{ 
        "type": "mongodb", 
        "mongodb": { 
        "db": "my_database", 
        "collection": "my_collection"
    }, 
        "index": {
        "name": "mainindex", 
        "type": "string",
        "bulk": {
            "concurrent_requests": 1
        }
    }
}'

Once the command is executed and I go to http://x.x.x.x:9200/_plugin/head/ I see the message: cluster health: yellow (1, 6).