There are black plastic guides that the cartridge slides on. The one on the right side toward the back should be flush with the front one. If not, it will prevent the cartridge from seating properly. If so, the tab on the top of the cartridge doesn't open the laser shutter all the way, causing the printer to believe the cartridge is not installed. These guides can be replaced, but it should be possible to jiggle or lift the cartridge past the rear guide to install the cartridge, which will enable it to work temporarily until it is replaced (provided someone doesn't remove the cartridge in the interim).