You are in Fastboot, but the bootloader is locked. That’s normal. Fix: If you want to flash custom images, you need to unlock the bootloader first using fastboot oem unlock or the official ASUS Unlock Tool.
Type the following command and hit Enter: adb reboot bootloader how to open fastboot mode on asus rog phone 3
: As soon as the device vibrates, release the Power button but continue holding Volume Up until the Fastboot/Bootloader menu appears. You are in Fastboot, but the bootloader is locked
Your ROG Phone 3 will immediately restart and open Fastboot mode. You are in Fastboot
: Press the Power button to confirm your selection.
Connect your ROG Phone 3 to your computer using a USB cable.