enable replicaSet in mongoDB yaml conf file
Check your mongod.log file, there should be line what lists all configuration items from you /etc/mongod.conf

2017-08-12T12:44:23.999+0300 I CONTROL  [initandlisten] options: { config: "/etc/mongod.conf", net: { port: 27017 }, replication: { oplogSizeMB: 1024, replSetName: "rs0" }, storage: { dbPath: "/var/lib/mongodb", journal: { enabled: true } }, systemLog: { destination: "file", logAppend: true, path: "/var/log/mongodb/mongod.log" } }


if you cannot see replication: { oplogSizeMB: 1024, replSetName: "rs0" } then it has NOT readed your current config file.

You can see what file it has readed when you look to start of that line config: "/etc/mongod.conf"

And remember, if your mongod is not running on standard 27017 port, you must adjust your commands to use the right port. Command rs.initiate() (without parameter), don't work if the port is something else than the default.

Update
If connecting to a specific port ensure you define it when using the client mongo --port 27117. For some reason the rs.initiate() command checks for a custom port based on the command line parameter and not what is defined in /etc/mongod.cof