To deploy React app, which was created using 'create-react-app --use-npm ', to Heroku, the following option need to be included:
heroku create <appname> --buildpack mars/create-react-app
If yo dont use those buildpack option, you might encounter Websocket error when opening the website using HTTPS instead of HTTP.
Information comes from https://create-react-app.dev/docs/deployment/#heroku