Your client does not have permission to get URL after deploying express API app on firebase hosting/functions

Question

I've created an Express API app. I want it to be served on firebase. I'm refering to this tutorial. As I understood this tutorial, it created new project for firebase and started writing APIs in it's index.js. But I wanted to point every API request to the APIs I created so I did a change in index.js of funtions folder

const functions = require("firebase-functions");
const app = require("../app");       //this is app.js of my express API app

exports.app = functions.https.onRequest(app);

I tried this on local with command firebase emulators:start. It is working fine on local but not working when deploying it on firebase.

Below is my folder structure

I'm getting

Your client does not have permission to get URL <code>/app/users/delete?id=1</code> from this server.</h2>

I searched this on SO and found few solutions like this.

As I am new to express and firebase both. Any help is appriciated to resolve this issue.

Thanks