npm run build
したあとに
The project was built assuming it is hosted at the server root. To override this, specify the homepage in your package.json. For example, add this to build it for GitHub Pages: "homepage" : "http://myname.github.io/myapp", The build folder is ready to be deployed. You may serve it with a static server: yarn global add serve serve -s build
みたいなのでるけど、yarn global add serve
で追加されてもパスがうまく通らないっぽい。
set PATH $PATH (yarn global bin)
これで追加。