He made the statement in an interview with Fox News on February 28 (US time) after returning from the second DPRK-USA summit in Hanoi without a agreement.
He said he wasn't satisfied and perhaps DPRK leader Kim Jong-un wasn't satisfied, too. Good relationship, but he decided that wasn't the right time to sign something, he added.
On March 1, US Secretary of State Mike Pompeo told a news conference in Manila, the Philippines, that nuclear negotiations with the DPRK will resume quickly.
He said the DPRK basically asked for the removal of all sanctions but were unclear about what they were prepared to offer at the Yongbyon facility.