google webmasters tells"found robots.txt file but can't download it"

Hello everyone,it’s the first time to use wordpress for me just two days ago.
Now,i use “XML-sitemap” create a sitemap.xml and try to setup in google webmasters tools.but it saids “We encountered an error while trying to access your Sitemap. Please ensure your Sitemap follows our guidelines and can be accessed at the location you provided and then resubmit” and “Network unreachable: robots.txt unreachableWe were unable to crawl your Sitemap because we found a robots.txt file at the root of your site but were unable to download it. Please ensure that it is accessible or remove it completely“.
I have try to use “KB Robots.txt” edited the robots.txt,but it does not work.
The root directory does not exist robots.txt.In XML-sitemap’s option has a word “The virtual robots.txt file generated by WordPress already enabled, make sure that the the blog document has no original robots.txt file”.
I don’t know what virtual robots.txt is,it can be closed?
How to do ?
I am chinese,certainly there is a syntax error,and it’s the first time seek help in foreign forum hope you will help me.