Inability to create units

[phoviper]

So I'm new to Civ and ran into something that doesn't make sense to me. I can create a musketman so long as I have 1 Niter, as long as I have an encampment, correct? Well, according to the image below I can't.
http://images.akamai.steamuserconte...710/2B8A6291AE23045056EA992E4CBF0323AC2858F5/
This isn't a big deal, just annoying.

 

[Browd]

You need to mine the Niter in Ravenna. The top of the screen shows that the only strategics you have connected are 1 Horse and 1 Iron, but no Niter.