Issue
Running heroku ps:scale web=1 (for example) is showing the error Couldn't find that process type
Resolution
"Process types" refer to the labels given to different kinds of processes as defined in your Procfile. You can read more about those here: https://devcenter.heroku.com/articles/procfile
If you don't have a Procfile, then you will be running the default process types that come with your application's buildpack: https://devcenter.heroku.com/articles/buildpacks
For example, the Ruby buildpack will check to see if Rails is installed and if so, will define the following process type:
web: bundle exec bin/rails server -p $PORT -e $RAILS_ENV
Problems can occur when using multiple buildpacks as they are order-dependent. That means that the last buildpack in the list should be the one that runs the webserver process. If you have your language buildpack at the top of the list (shown when running heroku buildpacks) then you won't get the benefit of default process types - this may result in the Couldn't find that process type error.
To fix
Remove the existing buildpacks with heroku buildpacks:clear and add them again in the right order using the heroku buildpacks:add with the --index option, making sure that the language buildpack is the last in the list.
You will need to add an empty commit and redeploy for the changes to take effect:
git commit --allow-empty -m "Adjust buildpacks on Heroku"
git push heroku master