The option to select the operating system in the BIOS is available only for Pinnacle Canyon and Grass Canyon NUCs. 6gen NUCs (Swift Canyon) the option is not available.
It is recommended to create a USB bootable drive (formatted as GPT) with Windows® 7 using our recommendations and NUC BIOS with default settings.
Windows 7* USB 3.0 Creator Utility
Latest BIOS 0036
This tread is very helpful, post 9