why does postman throws 500 internal server error when sending POST request?

Question

It's my first time building user auth using railway MySQL database and when I make a POST request in Postman, it returns 500 Internal Server Error.

I write the code in 3 files: userRepository.js, userService.js and userController.js.

Here is userRepository.js:

  const User = require('../models');
  const createUser = async (data) => {
    return User.create(data)
  };

  const getUserByUsername = async (username) => {
    return User.findOne({
      where: {
       username: username,
      },
    });
  };
  
  const getUserByEmail = async (email) => {
    return User.findOne({
      where: {
        email: email,
      },
     });
  };
  
  module.exports = {
    createUser,
    getUserByUsername,
    getUserByEmail,
  };

Here is userService.js

const userRepository = require('../repositories/userRepository');
const bcrypt = require('bcrypt');
const jwt = require('jsonwebtoken');

const registerUser = async (data) => {
  const { username, email, password } = data;
  
  const existingUserByUsername = await userRepository.getUserByUsername(username);
  if (existingUserByUsername) {
    throw new Error('Username already exists');
  }

  const existingUserByEmail = await userRepository.getUserByEmail(email);
  if (existingUserByEmail) {
    throw new Error('Email already exists');
  }

  const hashedPassword = await bcrypt.hash(password, 10);
  const user = await userRepository.createUser({
    username,
    email,
    password: hashedPassword
  });

  return user;
};

const loginUser = async (data) => {
  const { username, password } = data;

  const user = await userRepository.getUserByUsername(username);
  if (!user) {
    throw new Error('User not found');
  }

  const isPasswordValid = await bcrypt.compare(password, user.password);
  if (!isPasswordValid) {
    throw new Error('Invalid password');
  }

  const token = jwt.sign({ userId: user.id }, 'JWT_SECRET');
  return token;
};

module.exports = {
  registerUser,
  loginUser
};

And here is userController.js:

const userService = require('../services/userService');

const register = async (req, res, next) => {
  try {
    const user = await userService.registerUser(req.body);
    return res.status(201).json({
      status: 'success',
      message: 'User created successfully',
      data: user
    });
  } catch (err) {
    if (
      err.message === 'Username already exists' ||
      err.message === 'Email already exists'
    ) {
      return res.status(409).json({ 
        status: 'error',
        error: err.message
      });
    } else {
      return res.status(500).json({ 
        status: 'error',
        message: 'Internal Server Error',
        data: err
      });
    }
    next(err);
  }
};

const login = async (req, res, next) => {
  try {
    const token = await userService.loginUser(req.body);
    return res.json({ 
      status: 'success',
      message: 'User logged in successfully',
      data: token
    });
  } catch (err) {
    if (
      err.message === 'User not found' ||
      err.message === 'Invalid password' 
    ) {
      return res.status(401).json({ 
        status: 'error',
        error: err.message
      });
    } else {
      return res.status(500).json({ 
        status: 'error',
        message: 'Internal Server Error',
        data: err
      });
    }
    next(err);
  }
};

module.exports = {
  register,
  login
};

Any help would be appreciated. Thank you.