Javascript – I am unable to connect MongoDB properly to my server (node.js and express), would appreciate some guidance

When I send an API request using Hoppscotch, I get an error in my terminal and status code: 500 on hoppscotch. I am not able to rectify the problem. I am attaching my code below.

var mongodb = require('mongodb');
var ObjectID = mongodb.ObjectId;
var crypto = require('crypto');
var express = require('express');
var bodyParser = require('body-parser');
const exp = require('constants');

var genRandomString = function (length) {
    return crypto.randomBytes(Math.ceil(length / 2))
        .toString('hex')
        .slice(0, length);
};

var sha512 = function (password, salt) {
    var hash = crypto.createHmac('sha512', salt);
    hash.update(password);
    var value = hash.digest('hex');
    return {
        salt: salt,
        passwordHash: value
    };
};

function saltHashPassword(userPassword) {
    var salt = genRandomString(16);
    var passwordData = sha512(userPassword, salt);
    return passwordData;
}

function checkHashPassword(userPassword, salt) {
    var passwordData = sha512(userPassword, salt);
    return passwordData;
}

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

const mongoose = require('mongoose');
mongoose.connect('mongodb://127.0.0.1:27017/hwmDatabase')
    .then(() => {
        app.post('/register', (request, response, next) => {
            var post_data = request.body;

            var plaint_password = post_data.password;
            var hash_data = saltHashPassword(plaint_password);

            var password = hash_data.passwordHash;
            var salt = hash_data.salt;

            var name = post_data.name;
            var email = post_data.email;

            var insertJson = {
                'email': email,
                'password': password,
                'salt': salt,
                'name': name
            };

            var db = client.db('hwmDatabase');

            db.collection('user')
                .find({ 'email': email }).count(function (err, number) {
                    if (number != 0) {
                        response.json('Email already exists');
                        console.log('Email already exists');
                    }
                    else {
                        db.collection('user')
                            .insertOne(insertJson, function (error, res) {
                                response.json('Registration successful');
                                console.log('Registration successful');
                            })
                    }
                })
        });

        app.post('/login', (request, response, next) => {
            var post_data = request.body;

            var email = post_data.email;
            var userPassword = post_data.password;

            var db = client.db('hwmDatabase');

            db.collection('user')
                .find({ 'email': email }).count(function (err, number) {
                    if (number == 0) {
                        response.json('Email not exists');
                        console.log('Email not exists');
                    }
                    else {
                        db.collection('user')
                            .findOne({ 'email': email }, function (err, user) {
                                var salt = user.salt;
                                var hashed_password = checkHashPassword(userPassword, salt).passwordHash;
                                var encrypted_password = user.password;

                                if (hashed_password == encrypted_password) {
                                    response.json('Login success');
                                    console.log('Login success');
                                }
                                else {
                                    response.json('Wrong password');
                                    console.log('Wrong password');
                                }
                            })
                    }
                })
        });

        console.log("Connection Open!")

        app.listen(3000, () => {
            console.log("APP IS LISTENING ON PORT 3000")
        })
    })
    .catch(err => {
        console.log("Error lmao")

        console.log(err)
    })

The package.json file is attached below:

{
  "name": "hwm_nodejs_mongodb",
  "version": "1.0.0",
  "description": "hwm backend",
  "main": "index.js",
  "scripts": {
    "test": "echo "Error: no test specified" && exit 1"
  },
  "author": "Underdogs_United",
  "license": "ISC",
  "dependencies": {
    "body-parser": "^1.20.2",
    "crypto": "^1.0.1",
    "express": "^4.18.2",
    "mongodb": "^6.3.0",
    "mongoose": "^8.0.1"
  }
}

This code is part of a task which was given to us (I am part of a team). I have to create a database using MongoDB using node.js as server. I used express as I am comfortable with it.
When I used an API testing tool to see if i can send data to my database, I was unable to do so. It showed status code:500 and rather than getting a JSON file, it got an HTML file, which is an error.
The error it showed on the terminal is as follows:
Connection Open!
APP IS LISTENING ON PORT 3000
TypeError [ERR_INVALID_ARG_TYPE]: The "data" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received undefined
at new NodeError (node:internal/errors:406:5)
at Hmac.update (node:internal/crypto/hash:104:11)
at sha512 (C:UsersBS JaglanDocumentshwm_nodeJS_mongoDBindex.js:16:10)
at saltHashPassword (C:UsersBS JaglanDocumentshwm_nodeJS_mongoDBindex.js:26:24)
at C:UsersBS JaglanDocumentshwm_nodeJS_mongoDBindex.js:46:29
at Layer.handle [as handle_request] (C:UsersBS JaglanDocumentshwm_nodeJS_mongoDBnode_modulesexpresslibrouterlayer.js:95:5)
at next (C:UsersBS JaglanDocumentshwm_nodeJS_mongoDBnode_modulesexpresslibrouterroute.js:144:13)
at Route.dispatch (C:UsersBS JaglanDocumentshwm_nodeJS_mongoDBnode_modulesexpresslibrouterroute.js:114:3)
at Layer.handle [as handle_request] (C:UsersBS JaglanDocumentshwm_nodeJS_mongoDBnode_modulesexpresslibrouterlayer.js:95:5)
at C:UsersBS JaglanDocumentshwm_nodeJS_mongoDBnode_modulesexpresslibrouterindex.js:284:15