“Remove paper from center tray” - If you ever see this message on one of the copy machines, it's referring to the output tray. If there's nothing in the output tray, then the gray arm for the paper sensor has fallen off. See the attached picture of where the gray arm should be. If it's fallen off, hopefully, you'll find it laying in the output tray or on the floor nearby. If you find it, it can easily be reattached. However, it is possible to put it on upside down, which would result in the same error message.