First of all, it is not compulsory to use robots.txt file! you only need to use them in case you don't want the search engines to crawl specific pages or directories of your website.
In your case, you are restricting search engines to crawl tags and users' directories hosted on the root. Now, any page inside this directory will give this error.
I also recommend using the Google webmaster tool and verifying your website. You can test Robots.txt file from there.