Current generation processors, in this case AMD's Ryzen, only support Windows 10 out of the box. Any attempts to run a Windows 7 or 8 installer on a Ryzen board will fail because USB support will be disabled. I don't know the specifics behind why this is, but one can only surmise this was intentional on Microsoft's part, and for a reason I'll get to in a bit.
Using a USB driver injection tool, like MSI Smart Tool or Gigabyte's Windows USB Installation Tool, allows you to pack your necessary USB drivers in alongside your "unsupported" Windows iso, letting you proceed with the installation like normal. Some injection tools require your destination USB device (aka your bootable flash drive) to already have a Windows installer flashed on. I like to use Rufus to create bootable USB media for all my OS installs.
After installation, a Win7/8 build on Ryzen will pretty quickly throw up an error regarding Windows Update. This is where it all clicks, and you realize the whole incompatibility thing was entirely Microsoft's idea. Windows Update will cease to function if it detects that you're running anything other than Windows 10 on Ryzen. An easy way to bypass this is to install Wufuc, a program that spoofs a compatible CPU and tricks the Microsoft servers into thinking you're running on older hardware.