Set up build folder in server files in React-Node Js.

You can use cross-env, a package that sets environment variables across platforms. First, install cross-env as a dev dependency:

npm install cross-env --save-dev

Then, modify your build script to use cross-env to set the BUILD_PATH:

"scripts": {
  "dev": "react-scripts start",
  "build": "cross-env BUILD_PATH=../server/public react-scripts build",
  "test": "react-scripts test",
  "eject": "react-scripts eject"
 },

Now, when you run the build script, it should properly set the BUILD_PATH environment variable across different platforms.