This message means that your partition on the USB key is not active. Use diskpart.exe to mark the partition as active. Open a cmd window and type:
2) list disks
3) select disk [the disk number of your USB disk]
4) list part
5) select part [the partition number of the main partition]
This will mark the MBR partition as active and will allow the NUC and other PCs to boot from it.