Here is my server.js file code, followed by the complete error message in the browser and after clicking the Submit button.

const express = require('express');
const bodyParser = require('body-parser');
const MongoClient = require('mongodb').MongoClient;
const app = express();

MongoClient.connect('mongodb+srv://star:wars123@clusterstar2.j9vfh.mongodb.net/myFirstDatabase?retryWrites=true&w=majority', {
        useUnifiedTopology: true
    })
    .then(client => {
        console.log('Connected to Database');
        const db = client.db('star-wars-quotes'); // "star-wars-quotes" is the db name
        const quotesCollection = db.collection('quotes');
    })
    .catch(error => console.error(error));

app.use(express.urlencoded({
    extended: true
}));
app.use(express.json());

app.get('/', (req, res) => {
    res.sendFile(__dirname + '/index.html');
});

app.post('/quotes', (req, res) => {
    quotesCollection.insertOne(req.body)
        .then(result => {
            console.log(result);
        })
        .catch(error => console.error(error));

});

app.listen(3000, () => console.log('Listening on port 3000'));

Below is the error seen:

ReferenceError: quotesCollection is not defined
    at C:\Users\user\Desktop\MERN\crud-express-mongo\server.js:28:5
    at Layer.handle [as handle_request] (C:\Users\user\Desktop\MERN\crud-express-mongo\node_modules\express\lib\router\layer.js:95:5)
    at next (C:\Users\user\Desktop\MERN\crud-express-mongo\node_modules\express\lib\router\route.js:137:13)
    at Route.dispatch (C:\Users\user\Desktop\MERN\crud-express-mongo\node_modules\express\lib\router\route.js:112:3)
    at Layer.handle [as handle_request] (C:\Users\user\Desktop\MERN\crud-express-mongo\node_modules\express\lib\router\layer.js:95:5)
    at C:\Users\user\Desktop\MERN\crud-express-mongo\node_modules\express\lib\router\index.js:281:22
    at Function.process_params (C:\Users\user\Desktop\MERN\crud-express-mongo\node_modules\express\lib\router\index.js:335:12)
    at next (C:\Users\user\Desktop\MERN\crud-express-mongo\node_modules\express\lib\router\index.js:275:10)
    at jsonParser (C:\Users\user\Desktop\MERN\crud-express-mongo\node_modules\body-parser\lib\types\json.js:101:7)
    at Layer.handle [as handle_request] (C:\Users\user\Desktop\MERN\crud-express-mongo\node_modules\express\lib\router\layer.js:95:5)