For ASUS motherboards with USB BFB feature on it, this is the recommned method…
Its normal that other than that, it fails due to security/verification issues on modified bios files.
Further guidance here:
[Guide] How to flash a modded AMI UEFI BIOS - BIOS/UEFI Modding / BIOS Modding Guides and Problems - Win-Raid Forum (level1techs.com)
EDIT: Theres reports of users with the same issue, sometimes a different model of USB drive will work, some on FAT, some FAT32… it can be picky sometimes.
Cut off AC for 5m, CLR CMOS, power AC again and try again.
I had a board that only after removing GPU and RAM sticks and removing CMOS battery, it worked… picky 
Last resource…do the mod yourself:
[HowTo] Get full NVMe Support for all Systems with an AMI UEFI BIOS - Special Topics / NVMe Support for old Systems - Win-Raid Forum (level1techs.com)
Good luck.