Probaby because this device is set to boot with UEFI, or Secure Boot is turned on.
Secure Boot must be turned off to use FOG to image this device - this is not optional.
And if you can enable a legacy mode temporarily to image with, you should be able to use your current setup. If not, and you’re stuck with using UEFI mode, then you’ll need to specify an .efi file as your boot file and see if it works. All of the boot files are located in /tftpboot/ there are several .efi files to choose from, just try them one by one till you find one that works. I’d suggest trying ipxe.efi first.