How to Fix Heroku Application Error For a Successfully Deployed React App
If your app is already deployed you can just install the https://github.com/mars/create-react-app-buildpack in heroku settings, if still not working. Try adding a procfile with web: bin/boot. It works for me and hope it works for you.
If you used create-react-app you should add a new object called engines. Inside of the engines object, you specify the versions for npm and node. To do this open up your terminal and type in:
npm -vPress enter
node -vPress enter
{ "name": "portfolio", "version": "0.1.0", "private": true, "engines": { "npm": "5.7.1",// here put your version "node": "9.5.0" // here put your version },... the rest of the file}