I don’t know about the Node.JS SDK, but in order to have the relevant crowd pick your issue up more easily, you should also probably assign it to the “Node.js SDK” category…
As a first lead, did you debug/log the err parameter (error) in the callback you pass to insert method to check if something else went wrong?
The error you get indicates that the bucket either hasn’t been opened or has been shutdown(). I’m not sure if internal errors are known to close the bucket, but you should also probably include the code relative to cluster and bucket opening.