This guides assumes Heroku-cli has been installed.
Heroku-cli can be installed by running: brew tap heroku/brew && brew install heroku

Bundler Bug

In my case, there was a Bundler bug because I wasn't using Ruby 2.6.0 and above.
Make certain that you are using Ruby Version 2.6.0 and above as there is a known bundler bug on Heroku

I switched to Ruby 2.6.3.
I use RVM, so I run rvm use 2.6.3 Make certain that Ruby 2.6.3 is installed.
If it isn’t, install using rvm install 2.6.3

Precompiling assets failed

To solve this one, make certain to run: RAILS_ENV=production assets:precompile Don’t forget to set ENV['RAILS_MASTER_KEY']
You can the above using heroku config:set RAILS_MASTER_KEY=cat config/master.key --remote production
I included —remote production because I have two seperate environments (staging and production) setup on Heroku.

Then you are the good to go.
The joy of solving these issues is something else.
 I can’t trade for anything else.