I am having the same problem, but I’m not specifying the CSS route. As far as I understand that’s not needed as the plugin will generate a CSS file from the HTML specified in ‘src’.
What is your config.devUrl value? It works for me with http://localhost:8000, but not if I use my dev or prod URLs.
Asked for help in this thread too Critical CSS plugin
Edit: Forgot to say that I’m running Sage in a docker container.