"In August it often rains in the United States."
Translation:En aŭgusto ofte pluvas en Usono.
I don't see why not... Most likely it simply isn't in the list of sentences Duo knows about.
i think you should be able to. they accepted my answers: "Aŭguste pluvas ofte en Usono"
I'd say it's for the same reason you wouldn't say "la Francio" (unless you were actually saying something like, say, "the France of yester-year").
/disclaimer: Mi estas komencanto.