"Cannot GET /" error message in heroku browser page - node.js

Question

I get an error where the heroku page in browser just return this error Cannot GET / and when I am console log , it is saying 404 error but i have no idea on the error, already change the HTTPS to HTTP in server index.js .Any there change that I need to make ? Already tried other solution also not working fine it still have an error with the server, how can i fix this. Below i attach server index.js and my package.json.
server index.js
const express = require("express");
const app = express();
const http = require("https");
const { Server } = require("socket.io");
const cors = require("cors");
const server = http.createServer(app);
const user = [];
app.use(cors());
const io = new Server(server, {
cors: {
origin: "http://testing.herokuapp.com/" + (process.env.PORT || 5000),
methods: ["GET", "POST"],
},
});
io.on("connection", (socket) => {
user.push(socket.id);
socket.on("join_room", (data) =>{
socket.join(data);
})
socket.on("send_message", (data) => {
let new_data = {user_id: data.id, msg: data.text};
socket.to(data.room).emit("receive_message", new_data);
})
});
server.listen(process.env.PORT || 5000, () => {
console.log("SERVER OK");
});
package.json
{
"name": "client",
"version": "0.1.0",
"private": true,
"dependencies": {
"#testing-library/jest-dom": "^5.16.4",
"#testing-library/react": "^13.2.0",
"#testing-library/user-event": "^13.5.0",
"react": "^18.1.0",
"react-dom": "^18.1.0",
"react-scripts": "5.0.1",
"socket.io-client": "^4.5.1",
"web-vitals": "^2.1.4"
},
"scripts": {
"start": "react-scripts start",
"build": "react-scripts build",
"test": "react-scripts test",
"eject": "react-scripts eject"
},
"eslintConfig": {
"extends": [
"react-app",
"react-app/jest"
]
},
"browserslist": {
"production": [
">0.2%",
"not dead",
"not op_mini all"
],
"development": [
"last 1 chrome version",
"last 1 firefox version",
"last 1 safari version"
]
},
"devDependencies": {
"autoprefixer": "^10.4.7",
"postcss": "^8.4.14",
"react-router-dom": "^6.3.0",
"tailwindcss": "^3.0.24"
}
}