Whenever you face an issue with your script always check jmeter.log file, it should normally contain the root cause or at least enough information to guess it.
If you're looking for a built-in JMeter way of load testing MongoDB you will need to add the next line to user.properties file:
not_in_menu
This way you will have MongoDB Source Config back and will be able to specify your MongoDB host, port and other connection parameters. Later in JSR223 Sampler you will be able to get db object like:
def db = MongoDBHolder.getDBFromSource('sourceName', 'databaseName')
or if you need to supply the credentials:
def db = MongoDBHolder.getDBFromSource('sourceName', 'databaseName', 'username', 'password')
More information: How to Load Test MongoDB with JMeter