"Les garçons écrivent des lettres."
Translation:The boys are writing letters.
The reason it was not accepted is because it's not what the sentence was asking for, in French. The sentence "les garçons écrivent des lettres" is saying "the boys write (some) letters." The sentence you wrote would only be accepted if the French sentence was asking for "les garçons écrivent les lettres."
The word "les" lets you know it's plural ("boys") and the word "des" lets you know it's plural ("lettres").
Kate_Joy, here's something that I located in a Google Search. You'll have to read down to the end, as there's an additional detail. Apparently a lot of people are having trouble with Motorola for this feature (usually folks seem to be missing it). Good Luck! https://android.stackexchange.com/questions/60487/adding-a-new-spell-checker-to-motorola-moto-g
This particular sentence was asking for "some letters," not "the letters" therefore that is why the sentence "les garçons écrivent des lettres" would not be translated as "the boys are writing the letters." If you wanted to use the translation "the boys are writing the letters" the French equivalent would be "les garçons écrivent les lettres." If there is any confusion between "des" and "les" in French, here are some helpful pages to look at:
Hope that helps :)