This is false. According to the White House, the number is based on the U.S. trade deficit with Canada and higher military spending by the U.S., including expenditures associated with the North American Aerospace Defense Command,

Mark Carney immediately rejected Trump's claim in the Oval Office that the U.S. doesn't "do much business" with Canada.

Carney has stressed that he was elected to specifically “stand up” to the U.S. president and that Canada is “in a once-in-a-lifetime crisis.”