KYIV — President Trump’s special envoy to the Ukraine War urged Ukrainian President Volodymyr Zelensky to sign the now-deferred mineral rights deal with Washington during the envoy’s visit to Kyiv last week, but Zelensky’s chief of staff insisted on the Friday Oval Office signing that went badly awry, a senior US official involved with the talks told The Post. After three days of intense negotiations, retired Gen. Keith Kellogg secured the long-sought agreement with Ukraine that would have provided Washington with 50% of all future proceeds from the European country’s lucrative critical and rare-earth minerals.