mongo save causes error. Insert doesn't. I want to use save

My client javascript:

xmlhttp.open("POST", "http://35.197.106.195");
xmlhttp.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
xmlhttp.send(JSON.stringify(myData));

"POST /HTTP/1.1" 502 380

I'm running server app by executing: node app.js . where app.js has the following code:

var port = 3000;
app.post("/", (req, res) =>

var buyer = new Buyer({
        email: buyerObj[0].email,
         .
         .
    }).save()
    .then(item => {
        console.log("saved to database")
        res.send("record has been saved");
    })
    .catch(err => {
        console.log("err = " + err);
        res.status(400).send("unable to save 
        to database" + err);
    });
  });

which I changed to 

var buyers = {
email: buyerObj[0].email,
.
.
};
mycollections.insert(buyers);
res.send("record saved");

This took care of the 502 error. I also had to add:
?authSource=admin to the connection string to make insert work.

I want to be able to use save() and not insert.