Via ‘search console’ Google is sending this notification along with an underlined message that getting this warning means “suboptimal ranking.”
You can see the official warning message from Google.
Since October 2014, Google has been informing the webmasters that it is not fair for the websites to block CSS or Java Scripts and now after monitoring for a long time it has started to issue warning message.
If you have a little knowledge about robots.txt, then you can easily fix the issue in quickest possible time. Here, I have managed some codes and if any of these codes is visible, you must remove them because that is the culprit that is blocking Googlebot for crawling.
After removing such codes, you must check if the problem is fixed or not. To check the same use Google’s Fetch and Render tool. If the problem has not been removed and Googlebot is still blocked, there will be further instructions how to manage everything. Additionally you can Robots.txt testing tool for identifying the crawling issues.