Javascript – Didn't get json data in express.js

index.js file

import dotenv from "dotenv";
import connectDB from './db/index.js' ;
import { app } from "./app.js";

dotenv.config({
    path:'./env'
})
connectDB()
.then(()=>{
app.listen(process.env.PORT || 8000,(req, res)=>{
    console.log(`server is running at port at ${process.env.PORT}`)
})
})
.catch((err)=>{
    console.log(err)
})

app.js file

import express from 'express';
import router from './routes/user.routes.js'

const app = express();

// routes declaration
app.use("/api/user/register", router)

export {app}

user.routes.js file

import { Router } from "express";
import { registerUser } from "../controllers/user.controller.js";

const router = Router()
try {
    router.route("/register").post(registerUser)
} catch (error) {
    console.log("Error at user.routes", error)
}

export default router

user.controller.js

import { asyncHandler } from "../utils/asyncHandler.js"

const registerUser = asyncHandler( async (req, res) => {
    res.status(200).json({
        message: "OK",
    })
})
export {
    registerUser
}

I’m new to backend and trying to access json data message Ok, the server at port 8000 is running fine but at http://localhost:8000/api/user/register I’m getting error Cannot POST /api/user/register , I’m trying in PostMan and using POST method