Cannot GET /post

I cannot see my server on my local machine. i get the following error: Cannot GET /post.
My client server is running fine. However I am not able to see my server side.

post.js

import express from 'express';

const router = express.Router();


router.get('/', (req, res) => {
    res.send('THIS WORKS!')
});

export default router;

index.js

import express from 'express';
import bodyParser from 'body-parser';
import mongoose from 'mongoose';
import cors from 'cors';

import postRoutes from './routes/post.js';

const app = express();

app.use('/posts', postRoutes);

app.use(bodyParser.json({ limit: '30mb', extended: true }))
app.use(bodyParser.urlencoded({ limit: '30mb', extended: true }))
app.use(cors());

const CONNECTION_URL = 'mongodb+srv://***:***@***.net/?retryWrites=true&w=majority';
const PORT = process.env.PORT|| 9999;

mongoose.connect(CONNECTION_URL, { useNewUrlParser: true, useUnifiedTopology: true })
    .then(() => app.listen(PORT, () => console.log(`Server running on port: ${PORT}`)))
    .catch((error) => console.log(error.message));