Besides Ukraine, has the Trump Administration withheld or postponed aid (foreign or military aid) to another country?
Here are a couple instances where aid was actually withheld for a period of time (there may be other cases that I have not found).
The Trump administration announced Friday that it has cut nearly all the money the U.S. had planned to spend on aid projects for the Palestinians this year —including money to address a humanitarian crisis in the Gaza Strip.
On Monday, the new president signed a presidential memorandum reinstating the "Mexico City" policy — barring U.S. aid from any group that provides or "promotes" abortion overseas.
There were also some cases where Trump had proposed or threatened cuts to foreign aid, but those cases never materialized.
President Donald Trump said Monday he will move to cut foreign aid to Guatemala, Honduras and El Salvador for failing to stop people “coming illegally into the U.S.”
U.S. President Donald Trump’s fellow Republicans in Congress on Tuesday assailed his proposed cuts in the diplomatic and foreign aid budget, making it unlikely the cutbacks in global health, peacekeeping and other programs will take effect.
And finally, as for an implied question of, "Has Trump withheld aid in the past, if so, why is it a big deal now?" (Note that I'm not saying that you are asking this, just that it is a reasonable next question.) The issue at hand now with Ukraine is not merely that he has withheld foreign aid to Ukraine that had been allocated by Congress, but also that his rationale for doing so appears to be one, for his own personal benefit, and two, counter to the national interests of the United States.