I'll be honest, I would never use the complex option as it is an incredible waste of credits to use.
However, my guess would be that you have already reached your maximum allotment for that station type.
I'm making it and then using credits to move it into an area that my alliance just moved into so I can run missions and the like without taking up much space, plus I'd like to just see everything the game has to offer.
...but, uh, as you can see from the attachments I have no stations added to the complex.
EDIT: FIGURED IT OUT! It just so happened that the station I was trying to add was literally just outside the 10 km max range (according to Google Maps at least). I wondered if it was a bug because I thought it was just within the max range.