AutoCAD Inventor :: Negative Extrusion Refuses To Stick?

I have a model in which we want to make lighter than the prototype turned out to be. In order to acomplish this i am removing a chunk of the metal in the lid(images attatched. I have completed the sketch seen in red and extruded it to prove that the loop is complete but when I try to reverse the extrusioni to cut from the lid dragging down the arrow as an example, it turns the effected area a darker red than normaly seen when you cut something. When I let go of the left mouse button it pops back up to the original surface having no effect and refuses to go benieth it and when I apply I get the error seen at the bottom of the attatched image. I do this sort of cutting often instead of digging down to the root sketch then dealing with the ripple effect of the changes we want to make.