Why is my React app not running on heroku?

Question

Heroku

I built the app with React Create App using redux. On local hosting everything works, on github.io the app works fine too https://nickeny.github.io/comments-on-React/
But when I try to build the app on Heroku I get a TypeError: Cannot read property 'addComment' of undefined, as if the application itself was written incorrectly.
package.json:

{
  "name": "my-app",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "@testing-library/jest-dom": "^5.11.6",
    "@testing-library/react": "^11.2.0",
    "@testing-library/user-event": "^12.2.2",
    "express": "^4.17.1",
    "path": "^0.12.7",
    "react": "^17.0.1",
    "react-dom": "^17.0.1",
    "react-redux": "^7.2.2",
    "react-scripts": "4.0.0",
    "redux": "^4.0.5",
    "web-vitals": "^0.2.4"
  },
  "scripts": {
    "start": "node server.js",
    "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"
    ]
  },
  "description": "This project was bootstrapped with [Create React App](https://github.com/facebook/create-react-app).",
  "main": "index.js",
  "repository": {
    "type": "git",
    "url": "git+https://github.com/nickeny/main.git"
  },
  "author": "",
  "license": "ISC",
  "bugs": {
    "url": "https://github.com/nickeny/main/issues"
  },
  "homepage": "./"
}