I'm not familiar with your printer specifically, but I've had this issue with an HP Designjet. The cover wasn't in alignment with the switch that indicates that the cover is closed.
Look for a part on the cover that pokes in, and that should help you locate the switch. Press the switch. If it now indicates that the cover is closed, it is probably an alignment issue. If it still indicates that the cover is open, there may be a problem with the switch itself, or its wiring.