We still don't know your exact issue, but I am guessing you don't have enough contiguous space... which means you have the free space, it just isn't all together in one place
You can use
Carbon Copy Cloner (CCC) (it is free) to clone your current system to an external HDD. This makes a bootable copy of your current system and files on the external drive.
Boot from that external drive by holding down the option key when you boot and select the external drive
Use Disk Utility to erase your internal HDD
Use Carbon Copy Cloner to restore your clone back to the internal HDD
Boot from the internal drive by holding down the option key and selecting the internal drive now
Run BootCamp Assistant again
Woof, Woof - Dawg