You've reached your maximum marching armies: 1! Build camps to unlock more apc's I get this message..

This is one of the confusing statements that the game makes sometimes. To get more APC's you build barracks. You can only have 4 max. They are built as you level up, with 1 as a class barracks. Check under 'military' in the build menu to see when you can build more barracks. And under 'Class' if you haven't built the class barracks yet.

Also, occasionally (for me anyway) the game glitches and tells you that you have all your armies marching when you don't. I need to restart the game to clear this.