How do I fix an Invalid Host Header error when deploying my react app to Heroku?

Question

I'm having difficulty deploying my react app to Heroku. It runs locally just fine.

I keep getting the error Invalid Host Header when I go to the page: remiix.herokuapp.com

My repo is: https://github.com/hopeatina/remiix

I've tried using just proxy in package.json: "localhost:3001/"

I've tried using concurrently, as described here

I don't know what else to do and would like any enlightenment as to what I'm doing wrong.