basic nodejs and express server setup not working - node.js

Question

This should be very simple, steps I took:
1)used vue cli3 to create vue project (that worked fine).
2)created a server folder on the root level
3)created an index.js file
4)created api/routes/users.js
I am getting in the browser at http://localhost:4000/api/add Cannot GET api/add.
Here is my index file:
const express = require('express');
const bodyParser = require('body-parser');
const cors = require('cors');
const app = express();
//middleware
app.use(bodyParser.json());
app.use(cors());
const users = require('./api/routes/users');
app.use('api/add', users);
const port = process.env.PORT || 4000;
app.listen(port, () => console.log(`Listening on port ${port}...` ));
Below this is my users.js file:
const mongodb = require('mongodb');
const router = express.Router();
router.get('/', (req, res) => {
res.send('hello');
})
module.exports = router;
Here is what my package JSON looks like, not sure if this somehow creates problems since I first made the frontend and then started the backend.
{
"name": "fastpo",
"version": "0.1.0",
"private": true,
"scripts": {
"serve": "vue-cli-service serve",
"start": "node server/index.js",
"dev": "nodemon server/index.js",
"build": "vue-cli-service build",
"lint": "vue-cli-service lint"
},
"dependencies": {
"axios": "^0.18.0",
"body-parser": "^1.18.3",
"cors": "^2.8.5",
"express": "^4.16.4",
"mongoose": "^5.4.11",
"morgan": "^1.9.1",
"nodemon": "^1.18.10",
"serve-favicon": "^2.5.0",
"vue": "^2.5.22",
"vue-compose": "^0.7.1",
"vuetify": "^1.3.0"
},
"devDependencies": {
"#vue/cli-plugin-babel": "^3.4.0",
"#vue/cli-plugin-eslint": "^3.4.0",
"#vue/cli-service": "^3.4.0",
"babel-eslint": "^10.0.1",
"eslint": "^5.8.0",
"eslint-plugin-vue": "^5.0.0",
"stylus": "^0.54.5",
"stylus-loader": "^3.0.1",
"vue-cli-plugin-vuetify": "^0.4.6",
"vue-template-compiler": "^2.5.21",
"vuetify-loader": "^1.0.5"
},
"eslintConfig": {
"root": true,
"env": {
"node": true
},
"extends": [
"plugin:vue/essential",
"eslint:recommended"
],
"rules": {},
"parserOptions": {
"parser": "babel-eslint"
}
},
"postcss": {
"plugins": {
"autoprefixer": {}
}
},
"browserslist": [
"> 1%",
"last 2 versions",
"not ie <= 8"
]
}

Answer 1

Forgot to add "/" here before api: app.use('api/add', users);
should be app.use('/api/add', users)