Short version: Mexico agreed to all their concessions months ago (before Trump's tariff threat), and Trump claimed they agreed to buy a bunch of farm goods but there's nothing in the agreement about that and Mexico says they never discussed it and they don't know what he's talking about.
Either Trump is clueless and doesn't know what Mexico already agreed to, or he got played into accepting something they already agreed to.
This is part for the course for Trump: make a bunch of threats and slap a tariff on something. Markets drop, people get pissed. He blames the democrats, Mexico, or anyone but himself and claims he is being treated unfairly; says he will push them around to get what he wants. Then he declares he got what he wanted, even if it has nothing to do with what he first complained about, cancels the tariff. Markets go up because stability is good for growth. Trump declares victory for "fixing" the problem he caused.