Q: Problem install windows 10 bootcamp in iMac 5k fusion drive

I have bought the new iMac 27 inch 5K display with fusion drive. It is OS X 10.11.2 now. I have tried to install Windows 10 Pro 64 bit using Bootcamp. The problem is when it came to the process of select partition bootcamp ,format partition in NTFS and click next. There is a message show 'We couldn't create a new partition or locate an existing one. For more information, see the Setup log files.'   I cannot go to next step from this problem. And I have to quit Windows 10 installation.

Please give me any advices what to do with this.