Why might Congress have had difficulty paying the war debt After the American revolution

Because they were in debt to the french because the french aided the us.

The correct answer to this open question is the following.

Congress had a difficult time paying the war debt after the American Revolution because the government in the form of Congress, could not raise taxes under the Articles of Confederation. The government was not able to tax the citizens. If Congress needed money it had to ask for it to the states, that remained sovereign and they indeed could collect money. The central government was weak and only could operate the Post Office or overseeing the relations with the Native American Indian tribes.