I keep getting this message " Youv'e reached your maximum armies, build camps to unlock more APC's...

You will have a maximum amount of APCs that you can send out depending on your level and the number of barracks you have built. The most you have have is 4. If you try to send out an APC to attack anything or gather for example when all of your APC's are already busy then you will get this default message.

This is a confusing message because it will always show no matter whether you can build another barracks or not. And it is incorrectly worded. It just means though that you don't have a spare APC to use at the moment. Wait until one returns.

Can I buy another apc?