I have just completed my MVP for a MERN app and have deployed to Heroku. I am following the deployment tutorial from: https://dev.to/hawacodes/deploying-a-mern-app-with-heroku-3km7

The deployment is fine, however the app starts then crashes with the following log:

2022-02-28T14:33:52.129152+00:00 app[web.1]: > ygo-n-sample-hand@1.0.0 start
2022-02-28T14:33:52.129152+00:00 app[web.1]: > concurrently --kill-others-on-fail "npm run server" "npm run start --prefix sample-front"
2022-02-28T14:33:52.129153+00:00 app[web.1]: 
2022-02-28T14:33:52.140525+00:00 app[web.1]: sh: 1: concurrently: not found
2022-02-28T14:33:52.350800+00:00 heroku[web.1]: State changed from starting to crashed

My scripts from package.json looks like this,

"scripts": {
    "build-sample-front": "cd sample-front && npm run build",
    "install-sample-front": "cd sample-front && npm install",
    "heroku-postbuild": "npm run install-sample-front && npm run build-sample-front",
    "server": "nodemon app.js",
    "dev": "concurrently --kill-others-on-fail \"npm run server\" \"npm run start --prefix sample-front\"",
    "start": "concurrently --kill-others-on-fail \"npm run server\" \"npm run start --prefix sample-front\""
  },

My dependencies, defined in package.json is as follows:

"dependencies": {
    "connect-mongodb-session": "^3.1.1",
    "cors": "^2.8.5",
    "dotenv": "^16.0.0",
    "ejs": "^3.1.6",
    "express": "^4.17.3",
    "express-session": "^1.17.2",
    "mongoose": "^6.2.2",
    "node-fetch": "^3.2.0"
  },
  "devDependencies": {
    "nodemon": "^2.0.15"
  }

At first I did not have the npm module: concurrently installed. I then installed it, but get the same problem. Any help is appreciated.