You have most likely removed the entire fuser cover door instead of just the plastic trim "fuser cap" as described in the user guide.
Look carefully at the illustration of the fuser cap in Chapter 5 of the user guide and ensure that your printer looks the same. Remove only the fuser cap, and not the entire fuser door which should remain attached to the printer. The fuser cap has a rough exterior finish and contains no cutouts while the fuser cover door has a smooth finish and has a cutout big enough for the paper to travel through when entering the duplex unit.