Hello there! Hmm, I don’t think this would have anything to do with the environment variables.
Make sure your React app builds successfully locally by running npm run build in your project directory. This will generate a build folder with your production-ready files. Does it work locally using npm run build?
Double check that your build folder contains an index.html file after running the build command locally. There’s also this (pretty old) forum post, that might be helpful.