Most new ASUS motherboards do not have the option to let users to turn off the UEFI secure boot. The “Secure Boot Enabled” is always greyed out and unable to change that option. Yet, we do have the option to disable the secure boot by clearing secure boot keys.
This is reversible so no need to worry about breaking the warranty or damaging the BIOS. BIOS’s Secure Boot menu should show Secure Boot state as “enabled” and Platform Key (PK) state as “loaded”
To disable the UEFI secure boot:
After you cleared the Secure Boot Keys, Secure Boot will be automatically disabled. You can set the OS Type to Other OS now.