If following Al's advice you will find that in bios, the LED is enabled, you may try other test. I'm assuming that your NUC is NUC8i7HVK or NUC8i7HNK. Press and hold Power Button and count 1001, 1002, 1003, 1004. Release the Power Button. NUC should boot into Power Button Menu. In this state, the Skull should be illuminated, irrespective to any settings in bios. In case that the Skull is not illuminated, check the cable connection inside the NUC.
Since I don't have NUC6i7KYK, I don't know if the same happens in this NUC too.
Thank you for posting in the Intel Community.
I would like to confirm the following details:
1. For you have installed the latest BIOS is installed and load the defaults. Only change BIOS settings if a RAID is installed (if used).
2. Install the latest LED Manager for the unit you have.
If you still have issues, we recommend to contacting Intel Customer Support directly via phone or chat.
NOTE: I am editing you posts to delete personal information.
Hope this helps.
Intel Customer Support Technician
Under Contract to Intel Corporation