Trump’s problem with the Iran nuclear deal negotiated by the Obama administration is that it’s working. According to both U.S. officials and the international body created to supervise the agreement’s implementation, Iran is complying.
Over the course of the presidential campaign, Trump insisted that the agreement was a disaster. It’s not at all clear that he has any idea what is/isn’t included in the deal and simply adopted that position as a way to attack President Obama. So when his advisors told him that it was working and failed to provide him with an excuse to abandon the agreement, apparently he had “a bit of a meltdown.”