I am not from Intel but I bought that NUC and had experiences which could be like yours, so I will try to help.
My NUC arrived with an early BIOS, and that BIOS would not let me type from any of my Microsoft keyboards. I had to go find some non-Microsoft keyboard, which let me press some necessary keys. One of the first things that I did was update the BIOS, and after that I was able to use the Microsoft keyboards. I also had a video issue which you may not be having - the early BIOS required me to hook up to a TV (it would not display on a computer display) and the later BIOS displayed on everything but it is often squished small into the center of the screen.
Regarding not being able to boot, I think there was a paper sheet pointing out that only limited versions of OS have been supported/tested. If you are familiar with BIOS and study the wording carefully, you realize that a key thing is that the BIOS may have been delivered to be booting UEFI (common with Windows 8 but possible with certain earlier versions of Windows), and that you may want to change some setting to boot Legacy for older OS (I think their initial support of Windows 7 was suggesting Legacy mode). In other words once you can use a keyboard, maybe you just need to enable Legacy mode then your device(s) will boot.