Secretary of State Marco Rubio emerged from talks with Ukrainian officials in Florida on Sunday to say that there was still "much work to do" on a proposal to end the war with Russia. Missing from the Ukrainian group was Andriy Yermak, President Volodymyr Zelensky's right-hand man and chief of staff who resigned on Friday amid a corruption investigation. Mr. Yermak had served as lead negotiator in recent talks with American officials. Neither Mr. Rubio nor Rustem Umerov, who led the Ukrainian delegation after Mr. Yermak's resignation, revealed details of the discussions, although both described the talks as productive. Steve Witkoff, a special envoy for President Trump, plans to travel to Moscow on Monday to meet President Vladimir V. Putin of Russia, according to a U.S. official.