I have been receiving an error whenever I "post" something using express and node. i can load the website, but it crashes once i submit my form data

This has been the result:

/Users/anishladdha/url_shortener/node_modules/mongoose/lib/drivers/node-mongodb-native/collection.js:185
          callback(new MongooseError(message));
                   ^

MongooseError: Operation `urls.findOne()` buffering timed out after 10000ms
    at Timeout.<anonymous> 

when i run this code:

const express = require('express');
const bodyParser = require('body-parser');
const request = require('request');
const https = require('https')
const { json } = require('body-parser');
const mongoose = require('mongoose');
const shortID = require('shortid');
const URL = require("./models/url.js")

const app = express();
app.set('view engine', 'ejs');

var mongoDB = 'mongodb://localhost/url_short';
mongoose.connect(mongoDB, {useNewUrlParser: true, useUnifiedTopology: true}).then(console.log("connected?"));
var db = mongoose.connection;
db.on('error', console.error.bind(console, 'MongoDB connection error:'));
db.once("open", function() {
  console.log("MongoDB database connection established successfully");
});
app.use(bodyParser.urlencoded({extended: true}));


//app.get('/') works
app.post('/', async function(req, res) {
    let sid = shortID.generate
    await URL.create({
        longUrl: req.body.user_url,
        shortUrl: sid,
    });
    res.render('index', {sid: sid});
});
app.listen(3000, function() {
    console.log('listening on port:3000');
});

any help would be appreciated!