Google Bot accesses the website according to its own algorithm, but we can control its data retrieval through robots.txt file located in the root directory of the website. Therefore, this message means that one or more CSS / JS files on the website are being blocked by robots.txt so Google Bot will not be accessible. This behavior may be because you are blocking certain folders.
Allow: /*.js$ Allow: /*.css$
And if more complete then we will have:
User-agent: * Allow: /*.js$ Allow: /*.css$
And after adding that line, you can rest assured that your website is no longer blocking Google Bot from viewing CSS and JS on your website.Read more :