Build Successful and App Deployed but then Application Error on Heroku Hosting

Question

Tried all the community solutions on this first and nothing is working. My app is working locally without any issues on port 4000.

The procfile has

web: node app.js 

for heroku to run

The packages.json has:

  "scripts": {
    "preinstall": "npm install pm2 -g",
    "start": "node ./node_modules/.bin/pm2-runtime app.js -i max"
 },

Also installed PM2 globally on local system.

Any help? I'd 5 hours of "Application Error" after deployment.