Hello Louis_Koh ,
1. You have not described your hardware configuration (i.e. Your NUC model, memory installed, SSD and HDD installed). Any way in the following link you will find detailed instruction how to make clean Windows 10 installation: Windows 10 Help Forums
2. I suggest that you shall disconnect any external drives connected to your NUC and even remove from NUC any additional drive, leaving only drive where the Windows is going to be installed.
3. Since you are installing Windows on computer with UEFI bios, skip on para 12 in the above tutorial.
4. Before starting installation, enter bios setting (press F2 during boot) and set bios to default settings: press F9 followed by "Y". Check if you can see in Bios Advanced>Devices your SSD? Next, press F10 followed bt "Y" to save the settings and exit from bios.