It must start with a leading slash. Enter a directory path relative to current theme, where critical CSS files are located (e.g., /css/critical). Inside that directory, "Critical CSS" will try to find, among others, any file named "{entity_id}.css", "{bundle_type}.css",  or "{url}.css" (e.g., 1234.css, article.css, my-page-url.css, etc). If none of the previous filenames can be found, it will search for a file named "default-critical.css". Please refer to the documentation for a complete list of filenames.