Do you already have a second partition on your drive? If you don't there are a few possible reasons for the problem:
- Boot Camp Assistant can't make the partition if your drive is configured as a Core Storage device.
- You have requested more space than there is contiguous space available for the Boot Camp volume.
- Your drive was not properly partitioned or formatted originally.
The first item can be answered in Disk Utility. If you see your drive is listed in Disk Utility like this:
Macintosh HD
Macintosh HD
where both entries appear the same. Then it is a Core Storage device. I don't know how to set up a Boot Camp partition with this arrangement.
The second item means you need to ask for a smaller Boot Camp partition. However, if that isn't possible then you need to repartition the drive to fix it.
The third item solution is obvious. You need to repartition the drive.
I wish I could be of more help, but I'm not yet that familiar with Core Storage devices. I know how to revert a core storage device to a standard device, but you would need to be comfortable working with the Terminal.