The tool is teilling you that there aren't enough pixels from which to retrieve fill content elsewhere in your image layer. Maybe you have an inadvertent selection, maybe the layer you're on is substantially transparent, maybe you have the wrong layer selected... It's hard to tell without more information.
It might be helpful if you'd screen grab your whole desktop at the moment of the error, save it as a JPEG or PNG, and post it here. That will give others the chance to see what you see. Someone may be able to spot what's wrong.
-Noel