Ukraine's Western supporters can propose a ceasefire but why would either side agree to one? Putin seems to be in it for the long haul, he probably knows Western support for Ukraine will gradually go down and then once Russia's military recovers a bit they can go back their original aim of taking all of Ukraine. For Zelenskyy there exists plenty of popular support for trying to take back territory, and unless the US says they'll completely throw them to the wolves and cut off all aid (which would make the US look completely terrible internationally), why would he agree to a ceasefire if he thinks Russia will go back to war once they recover anyway?