Changing the speed of a clip while it's in the cut wont work, speeding up shortens it, and slowing down makes it longer, which affects the surrounding clips, so the error you get is just showing you that conflict. So press f to match back to that clip, and it appears in your viewer where you see your in/out points etc. Then hit command/apple+j which brings up the speed adjustment window. Adjust your speed, and drop it back in the timeline.
I think you can also hit F11 and it will overwrite the clip for you.