If you install Windows 7 or older operating system, the BIOS settings must be set to Legacy.
If you install Windows 10, it is recommended to set the BIOS settings to UEFI.


A blog about Operation System Deployments as MDT, MECM / SCCM and Windows
If you install Windows 7 or older operating system, the BIOS settings must be set to Legacy.
If you install Windows 10, it is recommended to set the BIOS settings to UEFI.