robots.txt blocks CSS or JavaScript files

What this means

robots.txt disallows stylesheets or scripts that the site's pages load, for the user agent the crawl identified as. Google renders a page the way a browser does before deciding what it contains and how it behaves on mobile; with the CSS and JS unfetchable it judges layout, mobile-friendliness and any script-injected content against a broken, unstyled shell. The check only tests the asset URLs actually sampled during the crawl, so it proves these files are blocked rather than that no others are, and it is skipped entirely when no assets were collected.

How to fix it

Allow crawling of your CSS and JavaScript: remove the disallow rules covering those paths, or add explicit Allow lines for the asset directories. Blocking them protects nothing, since the files are already public to anyone loading the page.

Fix effort

This check needs a developer fix: it turns on a routing, hosting or infrastructure decision that no automatic edit can make safely.

Authoritative source

https://developers.google.com/search/docs/crawling-indexing/robots/create-robots-txt