You can only raise so many troops of each culture. 1 div per 10k soldiers, to be exact. Put your pointer over the manpower icon and you'll see something like North german: 20/18, showing you can support 18 north german divs but you have 20, leaving you unable to build more for the moment. Conver more pops or pick a new culture.
This is a bit late (I was out seeing spider man 2), but I posted this for another guy playing Prussia yesterday to show my starting budget:
This is a bit late (I was out seeing spider man 2), but I posted this for another guy playing Prussia yesterday to show my starting budget: