Greenland is a territory of Denmark. The government of Greenland replied, “We have a good cooperation with USA…Of course, Greenland is not for sale.” The Danish PM also unequivocally rejected the idea. Trump took offense to the Danish PM’s “nasty” response.

NPR – No Joke: Trump Really Does Want To Buy Greenland
The New York Times – Trump’s Interest in Buying Greenland Seemed Like a Joke. Then It Got Ugly.