I checked out your repository that you linked.
You need to run
npm run build
This will create the build directory for you. This is essentially the folder that you deploy for production. So if you're serving your site with apache or nginx, you want to point the server at /your/project/directory/build .
If you're serving your site with NodeJS, same thing. Serve the build directory, and it should automatically pick up the index.html file.
I hope this helps.