Windows installation

Using Boot Camp, i was able to download the windows installer. Once windows opens and you have to configure it before you install, it asks, "Where do you want to install Windows?" There are 5 options that pull up as installation locations and each one of them says "Windows can't be installed on drive 0 partition (1-5). Does anyone know how to fix this problem so I can choose one of these options as an installation location because each one is unavailable.

I have a iMac (Retina 4K, 21.5-inch, Late 2015) with 3,1 GHz Intel Core i5

I ran the (in april updated) ISO 64 bit file in bootcamp, as said before i was able to partition and download the windows installer. Although I can not download windows because I can't put it on any of the disks because of the format