bootcamp partition not found...

Hi,

I'm trying to install Win 8 on my iMac with OS X EL Capitan, but it's harder than expected...

I have the iso file on USB for Windows 8 64 bit , I launch Bootcamp and I follow the instructions creating a partition in my hard drive , I digit the serial number, the installation starts, but once it checks the partition where to install Windows it can't recognize it and even if I format it doesn't work... here enclosed the message appearing and where I stop ...

someone has a similar problem or can support me with this?

thanks in advance