Every time I try to pack my BSA for my mod in the CK I get this error...anyone know why?

Yeah, sorry, but you're going to have build your bsa manually using the bsa archiving tool (Archive.exe) in the /Skyrim/ folder. The integrated plugin in the CK is broken for a lot of people, self-included.

How do you go about doing it with that tool?

Just drag in the files you need to import. Click off the checkboxes in the top left that correspond to the types of files you're importing, and check off the top 3 selectable boxes in the top right. (I think the first ... two are greyed out? I'm not on my computer atm.)

Then File --> Save As... (name of your mod here.)

For example, if your mod's plugin filename is OptimusPrimal.esp, the bsa should be named OptimusPrimal.bsa, not OptimalOptimus.bsa.