Heroku status error 503 when deployed - How do i interpret the error codes?

I keep getting this error in my heroku logs after deploying the app.

I'm still pretty new to this so I am not exactly sure how to read the errors but I have tried checking videos or documentations with various solutions to errors, but none seem to fix my problem.

The App works fine when I npm start it locally.

2020-07-21T00:14:48.005621+00:00 heroku[web.1]: Process exited with status 0
2020-07-21T00:14:48.063048+00:00 heroku[web.1]: State changed from starting to crashed
2020-07-21T00:14:48.065696+00:00 heroku[web.1]: State changed from crashed to starting
2020-07-21T00:14:56.336931+00:00 heroku[web.1]: Starting process with command `npm start`
2020-07-21T00:14:58.630133+00:00 app[web.1]:
2020-07-21T00:14:58.630180+00:00 app[web.1]: > react-boilerplate@0.1.0 start /app
2020-07-21T00:14:58.630186+00:00 app[web.1]: > react-scripts start
2020-07-21T00:14:58.630187+00:00 app[web.1]:
2020-07-21T00:15:01.109832+00:00 app[web.1]: ℹ ｢wds｣: Project is running 
2020-07-21T00:15:01.110173+00:00 app[web.1]: ℹ ｢wds｣: webpack output is served from
2020-07-21T00:15:01.110259+00:00 app[web.1]: ℹ ｢wds｣: Content not from webpack is served from /app/public
2020-07-21T00:15:01.110327+00:00 app[web.1]: ℹ ｢wds｣: 404s will fallback to /
2020-07-21T00:15:01.110510+00:00 app[web.1]: Starting the development server...
2020-07-21T00:15:01.110511+00:00 app[web.1]:
2020-07-21T00:15:01.202744+00:00 heroku[web.1]: Process exited with status 0
2020-07-21T00:15:01.245183+00:00 heroku[web.1]: State changed from starting to crashed

Index.js

import React from "react";
import ReactDOM from "react-dom";
import "./index.css";
import App from "./App";
import * as serviceWorker from "./serviceWorker";

ReactDOM.render(
    <React.StrictMode>
        <App />,
    </React.StrictMode>,
    document.getElementById("root")
);

serviceWorker.unregister();

Package.json

{
  "name": "react-boilerplate",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "@testing-library/jest-dom": "^4.2.4",
    "@testing-library/react": "^9.5.0",
    "@testing-library/user-event": "^7.2.1",
    "react": "^16.13.1",
    "react-dom": "^16.13.1",
    "react-router-dom": "^5.2.0",
    "react-scripts": "3.4.1"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",
    "eject": "react-scripts eject"
  },
  "eslintConfig": {
    "extends": "react-app"
  },
  "browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
    "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
    ]
  }
}