On Heroku: Command "webpack" not found

It's a buildpack thing.

Fixing with Heroku command line:

heroku buildpacks:add --index 1 heroku/nodejs --app brownfield-dashboard-qa
heroku buildpacks:add --index 2 heroku/ruby --app brownfield-dashboard-qa

Fixing with Heroku Web Dashboard

Go to the settings tab, and under Buildpacks, make sure that you have:

• heroku/nodejs
• heroku/ruby

That's what the working version has.