Can't figure out why Heroku website is outputting `cannot GET /` message - node.js

Question

so, it just says 'cannot GET /' everytime i try to view my app
here is the code
server.js
//Import requiered packages
const express = require('express');
const {Client} = require('pg');
//Create the conection to the postgres server
const client = new Client({
connectionString: process.env.DATABASE_URL
});
client.connect();
//Create the express app
const bodyParser = require('body-parser');
const app = express();
// parse application/json
app.use(bodyParser.json());
//Handle a post request at /query
app.post('/query', (req, res) => {
res.setHeader('Content-Type', 'application/json');
console.log("Receiving request");
if(req.body.query) {
console.log(req.body.query);
client.query(req.body.query, (err, r) => {
if (err) throw err;
rows = [];
for(let row of r.rows){
rows.push(row);
}
response = JSON.stringify(rows);
console.log(response);
res.end(response);
});
}
});
const port = process.env.PORT || 8080
//Start listening
const server = app.listen(port, function () {
console.log("App listening at ${host}")
});
package.json
{
"name": "haha",
"description": "joseph = obesity > lawa",
"main": "server.js",
"scripts": {
"start": "node server.js"
},
"dependencies": {
"express": "4.x.x",
"pg": "7.x.x",
"body-parser": "1.18.x"
}
}
I literally have no clue what's wrong and theres almost no erroirs in the CLI.
The errors in the CLI are basically irrelevant to this, as even if tehre are no errors im still presented with the 'cannot GET /' message
please help me :(