Hegseth said, and I quote:
"we must start by recognizing that returning to Ukraine's pre-2014 borders is an unrealistic objective"
This is a violation of Ukrainian sovereignty, the thing he said in the sentence before that he wanted to uphold.
You don't broker a peace deal by just letting an invader keep the disputed territory they currently occupy. IF Ukraine were agreeable to it, they could perhaps look at exchanging this territory for a (significant) material price, or other concessions such as Russian demilitarization. But you don't just start from the position "well that's lost already, suck it up"