MEXICO CITY (Reuters) - Mexico and the United States will be stuck at an impasse unless they solve U.S. President Donald Trump’s “unacceptable” demand of payment for a border wall, Economy Minister Ildefonso Guajardo said on Friday. Fresh home from failed talks in Washington, Guajardo also expressed concern about Trump’s “unpredictability” and habits on Twitter. President Enrique Pena Nieto on Thursday canceled a scheduled visit to the United States next week following Trump’s insistence that Mexico “respect” the United States and pay for a new wall on the southern U.S. border.  “Sincerely, there is clear disagreement at the moment because of a campaign promise he made that from the Mexican perspective is unacceptable,” Guajardo told Mexican television network Televisa.  “It is serious, because it leaves us at an impasse until we find a mechanism to resolve it,” the minister said, adding that  Trump should not underestimate Mexico’s resolve on the issue.  “Mexicans are not going to pay.” He said communication with Trump’s team remained open.   The White House on Thursday floated the idea of imposing a 20 percent tax on goods from Mexico to pay for a wall at the southern U.S. border, sending the peso tumbling and deepening the crisis between the two neighbors. Guajardo said the United States and Mexico could reach mutually beneficial improvements to the North American Free Trade Agreement if the issue of paying for the wall was resolved.   Trump had tweeted on Thursday that it would be better for the Mexican leader not to come to Washington if Mexico would not pay for the wall. Guajardo joined Foreign Minister Luis Videgaray in meetings with White House senior officials on Wednesday. He said that the meetings had been productive, but that Trump’s later tweet had forced Mexico to cancel due to the question of “dignity.”  “That unpredictability of reaction creates problems,” Guajardo said, referring to Trump’s Thursday tweet even as he praised a constructive attitude of other members of Trump’s team including Trump strategist Stephen Bannon.  “Their attitude to work, to plan, to see solutions is extraordinary, Believe me” Guajardo said.  Trump kept up his criticism of Mexico on Friday saying the United States’ southern neighbor had taken advantage of it long enough and demanding that “Massive trade deficits & little help on the very weak border must change NOW!” 