Apple's Mac Pro only supports installations of Windows 8 or later with Boot Camp, according to an updated Apple Support document that lists versions of Windows compatible with the 2013 Mac Pro



From MacRumors:

Boot Camp Assistant on the Mac Pro also specifies that it only includes support for Windows 8 or later, as evidenced in a screenshot from Twocanoes Software (via MacWindows) indicating that users are not able to install earlier versions of Windows. The 2013 Mac Pro is the first Mac that does not include support for Windows 7 with Boot Camp 5. It is unclear why Apple has chosen to drop support for Windows 7 on the Mac Pro, but it could be a sign that the company intends to discontinue support for the operating system in future Macs given its advanced age.

  Apple Drops Windows 7 Boot Camp Support in New Mac Pro