I spent lot of time today debugging this message when setting up rocketchat. I was sure I am setting MONGO_OPLOG_URL env variable correctly, but this this package kept complaining. So I followed your comment at

> db.runCommand({isMaster:1})
{
	"ismaster" : false,
	"secondary" : false,
	"info" : "Does not have a valid replica set config",
	"isreplicaset" : true,
	"maxBsonObjectSize" : 16777216,
	"maxMessageSizeBytes" : 48000000,
	"maxWriteBatchSize" : 100000,
	"localTime" : ISODate("2020-02-18T16:15:35.063Z"),
	"logicalSessionTimeoutMinutes" : 30,
	"connectionId" : 53,
	"minWireVersion" : 0,
	"maxWireVersion" : 8,
	"readOnly" : false,
	"ok" : 1,
	"$clusterTime" : {
		"clusterTime" : Timestamp(1582041152, 1),
		"signature" : {
			"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
			"keyId" : NumberLong(0)
		}
	},
	"operationTime" : Timestamp(0, 0)
}
> rs.conf()
2020-02-18T16:15:47.876+0000 E  QUERY    [js] uncaught exception: Error: Could not retrieve replica set config: {
	"operationTime" : Timestamp(0, 0),
	"ok" : 0,
	"errmsg" : "no replset config has been received",
	"code" : 94,
	"codeName" : "NotYetInitialized",
	"$clusterTime" : {
		"clusterTime" : Timestamp(1582041152, 1),
		"signature" : {
			"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
			"keyId" : NumberLong(0)
		}
	}
}

So as you can see problem isn't because of wrongly specified database in env variable, but replication set not being initialized. Your code should be fixed to handle such state, because as of now it guides user to incorrect thing.