Install Windows 7 or later is greyed out in BootCamp
After installing a fresh copy of El Capitan on my late 2013 13 inch MacBook Air I ran Boot Camp. I have a Windows 10 .iso on a DVD and a blank external USB drive connected to the laptop. BC recognized the .iso and the external hard drive. The problem is the third option install or remove windows 7 or later version is greyed out.
What can I do so that option is not greyed out?
MacBook Pro (15-inch Late 2011), OS X El Capitan (10.11.6)