"I see the border between the countries."
Translation:Veo la frontera entre los países.
As cr48laptop has said, nations and countries are different words. They usually mean the same thing, but a country is more of a geographic entity, and a nation is more of a people who share a heritage. For example, the Sioux nation is in the country of the United States. So while they are usually synonyms, they can have subtly different meanings.
Because "naciones" means nations, and the question asked for countries. They mean the same thing, yes, but Duolingo wants "paises".
I misread as "I can see the border...." And wrote Puedo ver la frontera entre de los países oops