Just in case anyone else is receiving an ‘Application Error: An Error occurred in the application and your page could not be served’ error after pushing your rails project my fix was pretty simple.
In the console I ran:
$ heroku run rake db:migrate
This gave me an error saying that because Heroku didn’t support SQLite3 at a production level I needed to use postgresql, however I had forgotten to include the gem in the gemfile.
Included gem ‘pg’ in my gemfile
$ bundle update
$git add -A
$git commit -m “add gem ‘pg’ to gemfile”
$git push heroku master
Came back working!