django-react-boilerplate: bin/run_webpack: line 15: npm: command not found while pushing to heroku

I follow all the README Steps for running it local. running locally fine (migration/run-server) Then

heroku create appname
git add -a
git commit -m 'intial commit'
git push heroku master

has below error;

Counting objects: 87, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (70/70), done.
Writing objects: 100% (87/87), 22.28 KiB | 0 bytes/s, done.
Total 87 (delta 5), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote: 
remote:  !     Warning: Multiple default buildpacks reported the ability to handle this app. The first buildpack in the list below will be used.
remote:                         Detected buildpacks: Python,Node.js
remote:                         See https://devcenter.heroku.com/articles/buildpacks#buildpack-detect-order
remote: -----> Python app detected
remote: -----> Running pre-compile hook
remote: ----->Pre-compile hook
remote: -----> Running Webpack
remote: jquery-webpack-stats.json created
remote: webpack-stats.json created
remote: bin/run_webpack: line 15: npm: command not found
remote:  !     Push rejected, failed to compile Python app.
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !       Push rejected to meddy.
remote: 
To https://git.heroku.com/meddy.git
 ! [remote rejected] master -> master (pre-receive hook declined)

and the app.json file is ;

{
  "name": "appname",
  "description": "Meddy Heroku app.",
  "scripts": {
    "postdeploy": "python manage.py migrate"
  },
  "env": {
    "ALLOWED_HOSTS": {
      "description": "Django ALLOWED_HOSTS setting, e.g.: .appname.herokuapp.com"
    },
    "DISABLE_COLLECTSTATIC": {
      "description": "Heroku setting to disable Django collectstatic (it is run by bin/post_compile)",
      "value": "1"
    },
    "DJANGO_SETTINGS_MODULE": {
      "description": "Django settings Python import path",
      "value": "appname.settings.production"
    },
    "SECRET_KEY": {
      "description": "Django SECRET_KEY setting",
      "generator": "secret"
    }
  },
  "formation": {
    "web": {
      "quantity": 1,
      "size": "free"
    },
    "worker": {
      "quantity": 1,
      "size": "free"
    }
  },
  "addons": [
    {
      "plan": "heroku-postgresql:hobby-dev",
      "options": {
        "version": "9.5"
      },
      "as": "DATABASE"
    },
    {
      "plan": "heroku-redis:hobby-dev",
      "options": {
        "version": "3.2"
      },
      "as": "REDIS"
    },
    {
      "plan": "sendgrid:starter"
    },
    {
      "plan": "papertrail:choklad"
    },
    {
      "plan": "opbeat:test"
    }
  ],
  "buildpacks": [
    {
      "url": "heroku/nodejs"
    },
    {
      "url": "heroku/python"
    }
  ]
}

PS : I don’t want to host repo on git and deploy via Button.