I'm a native Italian doing this course just for fun. In Italian you don't have to express the subject of your sentence, you can omit it. Because verbs have a distinct conjugation that is different for each and every person. So, from the end (conjugation) of the verb, you can infer the subject without mistake! In Italian we sometimes do add it, sometimes we don't. It's mostly a matter of preference and naturalness, fluency while speaking. No set rule on that! Hope this was helpful.
I'm curious. Do you do your lessons on a cell phone or a computer and, if you do your lessons on a cell phone, can you see all the comments? There are currently 159 comments on this thread. This question has already been answered many times. The computer does not make mistakes. You are getting your answer marked incorrect because you did not write the correct answer. Most likely you you got the first word wrong by writing "Lo" instead of "Io/io". Let me know if I'm correct.