Userschema and controller

I am building a user signup and login api and admin signup and login using express and currently I am testing in the postman, but somehow postman keeps return "error": "firstName is not defined" even though I posted firstname etc. here is my code, can anyone help me to explain it what is wrong? I saw so many videos using all different kinds of method, like generateAuthtakoken in the user.model or joi password library, it is just so overwhelming, can you help me to point to a direction as to how to use express to create ?

this is my user.model file:

const mongoose = require("mongoose");
const bcrypt = require("bcrypt");
const validator = require("validator");

const userSchema = new mongoose.Schema(
  {
    firstName: {
      type: String,
      required: true,
      trim: true,
    },
    lastName: {
      type: String,
      required: true,
      trim: true,
    },
    email: {
      type: String,
      required: true,
      trim: true,
      unique: true,
    },
    password: {
      type: String,
      required: true,
    },
    role: {
      type: String,
      enum: ["user", "admin"],
      default: "user",
    },
    contactNumber: { type: String },
    profilePicture: { type: String },
  },
  { timestamps: true }
);

//static signup method
userSchema.statics.signup = async function (email, password) {
  //validation
  if (!firstName || !lastName || !email || !password) {
    throw Error("All fields must be filled");
  }
  if (!validator.isEmail(email)) {
    throw Error("Email is not valid");
  }
  if (!validator.isStrongPassword(password)) {
    throw Error("Password is not strong enough");
  }

  const exists = await this.findOne({ email });
  if (exists) {
    throw Error("Email already in use");
  }
  const salt = await bcrypt.genSalt(10);
  const hash = await bcrypt.hash(password, salt);
  const user = await this.create({ email, password: bcrypt.hash });
  return user;
};

//static login method

userSchema.statics.login = async function (email, password) {
  if (!firstName || !lastName || !email || !password) {
    throw Error("All fields must be filled");
  }

  const user = await this.findOne({ email });
  if (!user) {
    throw Error("Incorrect Email");
  }

  const match = await bcrypt.compare(password, user.password);

  if (!match) {
    throw Error("Incorrect password");
  }

  return user;
};

module.exports = mongoose.model("User", userSchema);

this is my controller file:
const User = require("../models/user");
const jwt = require("jsonwebtoken");

const createToken = (_id) => {
  jwt.sign({ _id }, process.env.JWT_SECRET, { expiresIn: "3d" });
};

//login user
const loginUser = async (req, res) => {
  const { email, password } = req.body;
  try {
    const user = await User.login(email, password);
    // create token
    const token = createToken(user._id);
    res.status(200).json({ email, token });
  } catch (error) {
    res.status(400).json({ error: error.message });
  }
  res.json({ msg: "login user" });
};

//signup user
const signupUser = async (req, res) => {
  const { email, password } = req.body;
  try {
    const user = await User.signup(email, password);
    // create token
    const token = createToken(user._id);
    res.status(200).json({ email, token });
  } catch (error) {
    res.status(400).json({ error: error.message });
  }
  res.json({ msg: "login user" });
};

module.exports = { signupUser, loginUser };

and my router file:
const express = require("express");
const router = express.Router();
const { signupUser, loginUser } = require("../controller/auth");
//login route
router.post("/login", loginUser);

//signup route
router.post("/signup", signupUser);

module.exports = router;