Yo is never required in Spanish as far as I have seen. I have seen "Yo estudié" offered as an alternative but never marked as an error. There may have been a simple little mistake with a different part of the sentence. I typed "i studied a lot about guatemala" in that exact manner with no capitals and no punctuation and was marked correct with no suggestions on this one. There may have been something simple that you would have to reread 6 times to find on the last one.