Here’s the kicker: Assuming Russia is willing to negotiate a deal, would it honor that deal? It did, after all, guarantee security in exchange for Ukraine relinquishing its nuclear weapons, and it broke that commitment.
Ukraine has very good reason to believe that Russia would only use a deal to stop the war as an opportunity to build its strength for another invasion, later. There’s strong evidence that it’s not the capture of separatist territories that is Putin’s goal, but the denial of Ukrainian as a distinct cultural identity, and to prevent it from aligning culturally with the West (even leaving aside the issue of NATO).
If you think the enemy won’t honor a deal, and won’t stop its aggression long-term—and Ukranian leadership has said that that’s exactly what they believe loudly and often—what’s the incentive to negotiate for a ceasefire?