Others already told you why that didn’t work.
There’s a number of ways in which you can make Go work with React. Personally, while developing I just npm start a development server for React, which proxies to my Go application for requests. For example, if I’m listening on port 8080 in my Go service, I’ll just add "proxy": "http://localhost:8080/" to package.json, run each separately and my web requests will be served by the Go backend.
On the other hand, in production I don’t want a React development server running, and I want to have one sole binary that I can easilly move, deploy and run as a service or whatever. That’s why I build the React project so it’ll package all css and js in a couple of minified files, then embed those at the Go end using go-bindata and serve them directly from Go. Building the Go project then will give you a nice self-contained binary with everything you need. That’s what my previous answer showed an example of.