Hi All -
I encountered the same problem as the OP. I have Arma 2 1.6 from steam. After getting this message when trying to buy units in the badlands mission, I messed around for a bit and finally got a workaround by doing the following:
1. Go to buy units dialog
2. SINGLE click the unit you want to build
3. Click the "Add empty" button at the bottom of the dialog.
This works and builds you a new vehicle. Why? The vehicle is delivered without any crew, so I think previous posters surmising that there were two many units on the map were correct. If you want crew for the new vehicles, assign some crew that you already have to the new vehicles. My squad seems to be full with the four guys from razor and 8 crew members from BRDMs I bought. I guess if you were hoping to have many vehicles built and crewed by non-razor members, then this might not help you. But.... just think of the nightmare you're saving yourself by not having to control that many crappy AI drivers! ;)