Where are you seeing that error? I'm confused. It sounds like it's coming from an add-on, so I don't know what it's reading or what it's checking for that. The add-on author would be best placed to clarify.
ElasticSearch is designed for use with multiple servers. That message just means you only have a single ES node (basically every shard is allocated, but your ES configuration says you want more than 1 copy of data, but it has no where to put that extra data).