I was able to get around it by pushing all files to heroku git instead of mentioned files in documentations

So, instead of,

git add -f Procfile config.json package.json build && git commit -am "adding Procfile and configs for Heroku"

I did,
git add . && git commit -am "adding Procfile and configs for Heroku"

This solved my problem.

However, I have moved away from Heroku and deployed my forum on GCP.