Mac OS X El Capitan BootCamp doesn't target Windows 10 ISO
Unfortunately your particular Mac does not seem to support Windows 10. You can check the link here to verify. The last supported version of Windows on your Mac is 8.1.

Some people have had success installing 8.1 and then upgrading to 10, but you will not get Apple supported drivers and installation might be problematic. Unless you have particular reasons for needing Windows 10 you may want to stick with Windows 7.