Here are a couple of facts about NATO, to pre-empt B.S. posts. NATO has two "Budgets." The first one is its $5-billion administrative budget. No member country was in arears for that budget when Trump made his comments, and none are now. The second "budget," is a formula agreed to by member states that they all would fund their own military at a rate of 2% of GDP by 2024. Since that date has not been reached, it would be impossible for anyone to be delinquent, as Mr. Trump incorrectly asserted many times. And, it would be incorrect for him to say that he made people who owed nothing to catch up on their payments.
More likely, the source for his comments comes from an old John Birch Society ongoing allegation that member nations did not pay their share of the cost of running the United Nations. The John Birch Society called for us to get out of the UN. This played well historically with Trump's base. Inserting NATO for UN was an easy switch for propaganda purposes.