Skylake USB ports need a driver to work.
If you are trying to load windows 7 via a usb connected drive, it will not work because windows 7 did not have the drivers.
The solution is to either slipstream the drivers into the install, or use a sata connected dvd drive to do the install.
Some newer motherboards have a bios option to temporarily use legacy support to allow you to boot.
Read your manual for such a bios setting