Google Bot visit the website under its own algorithm, but we can control it to retrieve data via the robots.txt file located in the root directory of the website. Therefore, this announcement means that a file or CSS / JS on websites are blocked by robots.txt, Google Bot will not be accessible. This phenomenon may be because you block certain directories.
And if more fully, we will have:
And after add that line, you can rest assured that your website is no longer block Google Bot see CSS and JS on the website again.