Altere sua string de conexão para
variable = 'mongodb://username:[email protected]:27017,developerbookcluster-shard-00-01-i548w.mongodb.net:27017,developerbookcluster-shard-00-02-i548w.mongodb.net:27017/test?ssl=true&replicaSet=DeveloperBookCluster-shard-0&authSource=admin&retryWrites=true&w=majority';
