What is the difference in Arabic between "an american boy" and "a boy is american".
Technically there isn't but it is a matter of logical meaning when the phrase is heard.
Given that the adjective here is proper (one of nationality), this is logically heard in Arabic as a phrase (implying 'there is an American boy...' and we will know more about him in the sentence) rather than an independent sentence (implying that all boys are American).
I guess this is how Duolingo works: no teaching (i.e. giving explanations), always testing, and learning comes by making mistakes in this testing mode. There is some merit to it, even though it can be frustrating. Oh, and then there are the people who provide explanations :-)
This term is offered as a text completion where the alternatives are also correct (an intended meaning is not given, so why is "an American boy" correct, but not "an American girl"?)