It doesn't even mater if the system can see the cabinet or not. You should still be able to add a card in Card assignments even if the cabinet exists or not.

What does the licensing info say about how many cabinets you can have? You have a BEXU, so you should be OK to 7 cabinets. Maybe the system does not see the BEXU card? Maybe re-seat the processors.

Also check the Maintenance - Hardware Info and control - CIX Processor. System type should say /BEX-" and look to see if the BEXS box is checked. Also when you hovered over Hardware Info, 7 Exp Cabinets should have showed up.

I notice that about a year ago you were having troubles adding new extensions after adding a BEXU. I don't know if you got that figured out or not.