HELP with DELL2155cn cartridge jam

I am a teacher and someone gifted me a DELL 2155cn printer for my classroom. When I went to change the ink cartridges, the black one was stuck. I have access all the color cartridges but the black one won't slide back, even when I unlock it. I am assuming it became jammed in transport, as it was moved without a box/ wrapping.

Any suggestions about how to unlock it so I can use the printer?

Thank you in advance