Power off and then hold the power button down for 4 seconds. This should result in a recovery menu being displayed. Use F2 there to enter into BIOS Setup.
Hope this helps,
I had the same issue before I have 2x Samsung 950 pro 256gb setup raid for my nuc6i7kyk but you need to download raid driver to USB flash drive and setup in bios , please follow this guild for raid 0/1 setup : RAID Setup Procedure for Intel® NUC Kit NUC6i7KYK
Thanks so much guys I think I made it!