XTU only supports X and K processors. You have a G processor. Your processor is not supported.
On the XTU download page, it tells you what processors are supported. It also says " Intel XTU may or may not work on unsupported processors."
Also, regarding undervolting"
My understanding is that, in response to Plundervolt, Intel has made changes to the microcode that alters or removes the capabilities for undervolting. You are not the only person who has lost a capability here, but Intel had no choice; vulnerabilities must be addressed.
Thus does Doc's "nothing is forever" get applied, unfortunately.