Sorry for the slow reply due to the Lunar New Year holiday and any inconvenience this may cause.
May I ask why you need to lower the voltage of the CPU? Is there any abnormality in use that leads to the need to undervolt?
It is recommended that you refer to the link below and install AI Suite 3 to start enabling Ai Overclocking.https://www.asus.com/support/FAQ/1043257/
If there is an inoperable part of the process, please let me know which is it.