mongodb: Can I create an unshard Replica set for a sharded cluster?
mongodb: Can I create an unshard Replica set for a sharded cluster?


As per MongoDB documentation here Starting in MongoDB 3.6, all shards must be replica sets. Before you can upgrade your 3.4 sharded cluster to version 3.6, you must convert any shards that are running as standalone mongod instances to replica set shards.


  Note: To converts a standalone shard to a single-member replica set shard. The procedure assumes that the
  single member runs on the same host and port as before.