Two Three problems:
1) Your processor is not supported by XTU. XTU only supports X and K processors.
2) Intel has disabled underbolting due to security concerns.
3) Had your laptop manufacturer done a better job of design and heat dissipation, you would not need to undervolt.
Doc (not an Intel employee or contractor)
You don't. It is not actually being undervolted; the microcode running on your processor does not support undervolting.
As Al has stated, Intel has disabled support for undervolting. This is across all existing processors and designs. It was the only mitigation for the Plundervolt! vulnerability that Intel was able to identify in the time available. For more information, see articles Plundervolt! A new Intel Processor ‘undervolting’ Vulnerability and What is Plundervolt! and How Does it Work? Could Intel have done better? Probably, but not in the timeframe in which a response was necessary.
I sympathise with folks that were using undervolting to correct an otherwise unacceptible thermal/acoustic situation. The fact is, however, that the *real* solution is to purchase a system (or cooling solution) that properly supports the thermal situation - and does so without acoustic penalty. That system (especially laptop) manufacturers are daily screwing their customers by providing only crap solutions borders on criminal. Worse, they do slimy things like enforce system throttling settings for no other reason than to make their battery numbers (artificially) look better. Consumers, do your homework!