I can build elsewhere, so it's not a normal build bug ..
I took a screenshot (What a surprise, eh? ;-))

The red part is the area i've tried to build .. it comes up with problem (3) Too Close to Enemy Army ..

members.optusnet.com.au/~playteq/whywhywhy.jpg

As you can see, there's no enemy within like, 10 hexes.
(I've covered "Player-X's" troops as to not give away info etc.)

So why the heck can't I build?!

yeap...i once had that problem
i thought they were hiding somewhere...so i switch the view but no enemy is around
i think its the spot where some enemy died of nearby which isnt cleared...