Insert data in mongodb using Socket.io
It's because you are passing a string data, to your insertData function (judging by your code, I guess you are supposing that data is actually an object).
Then you are giving data.msg, which is obviously undefined, as the first argument of the save methods, which is expecting you to provide a valid object here.
Please also note that you are using the save method on a newly created document, instead of using insertOne.
As you can see in the api documentation, using save is now deprecated. (I guess you are using the last version which is currently 2.1)