I can help you out as this issue DROVE ME NUTS.
1. I didnt have a Robots.txt
2. I had Yoast installed
3. Im pretty sure it created a Robots.txt even though it doesnt exist in my root (.com/here)
4. My Google webmaster tools shows this
THIS IS HOW YOU FIX THIS ISSUE. Create a Robots.txt and only add this (or whatever you want. I keep it simple and rank well/have no issues at all. You CANT trick google)
1. login to wordpress
2. Click SEO in your side toolbar (Yoast WordPress Plugin settings).
3. Go to edit files under SEO (in the side toolbar)
And now you have the option to edit your Robots.txt file.