How to add JWT token authentication to protect routes

I've added the required code but still I'm getting:

Cannot GET /api/login

I'm testing this on Postman as well as directly on browser. However I still haven't fully implemented JWT authentication. But other api end points like api/articles, api/update, api/delete etc are working fine. Here is my code.

server.js

const express = require('express');
const bodyParser = require('body-parser');

const port = (process.env.PORT || 3000);
const app = express();

const api = require('./routes/api');
const cors = require('cors');

app.use(bodyParser.json());
app.use(cors());

app.use('/api', api);
app.get('/', function(req, res) {
    res.send('Server is up and running!');
})

app.listen((3000), function() {
    console.log('Server listening on PORT ' + port)
});

api.js

...
const jwt = require('jsonwebtoken');

router.post('/login', (req, res) => {
    const user = {
        id: 1,
        username: 'tanzeel',
        email: 'tanzeel@fakemail.com',
        password: 'fakepassword' 
    }

    jwt.sign({ user }, 'secretKey', (err, token) => {
        res.json({
            token
        })
    })
})

package.json

...
"dependencies": {
    "body-parser": "^1.19.0",
    "cors": "^2.8.5",
    "express": "^4.17.1",
    "jsonwebtoken": "^8.5.1",
    "mongoose": "^5.6.13",
    "serve": "^11.3.2"
}

I've other routes as well but I'm not mentioning here as they are working perfectly and returning the right json response.

Please point out my mistake.