Cannot select system partition

[sonicserve]

I want to boot both BIOS-based and UEFI-based computers. So on my USB hard drive I created a System FAT partition(350M) and a NTFS boot partition. But it does not allow me to select the system partition. There is no radio button for the system partition. What is the problem?

I am using WinToUSB Free v3.1. I am trying to install Windows 10 Pro 64 bits. My USB hard drive is using MBR partition.

Screenshot is attached.