"This person has a blue watch."
Translation:Αυτός ο άνθρωπος έχει ένα μπλε ρολόι.
I thought it might be something like that, but of course it’s impossible to guess correctly.
άτομο, like just about all nouns ending in -ο, is neuter, not masculine.
Thus “this person” is αυτό το άτομο, not αυτός ο άτομο(ς).
If the noun and the gender of the words before it don’t match, Duo can’t tell whether you used the wrong noun or the wrong gender of other words. It generally guesses that the first word (e.g. the article) is correct and the second word (e.g. the noun) is wrong.
Things like this are why you should always quote the entire sentence, because the error is often not where you think it is, but in some other word.
Let me answer the second part of your comment first:
You give feedback anonymously. The report that is sent to the incubator, where forum moderators have no access. Course contributors can see the reports but cannot match them to the users who submitted them. Actually, jaye has been building a halo over their head for quite a while now trying to match reports to user comments in the sentence discussions, based on the details left in the comments and how well the timing between them matches. I'm not sure any of us can really appreciate the effort and dedication this takes considering there are no tools for doing this.
As for using Αυτός ο vs αυτό το, it has to do with what follows, e.g. αυτός ο άνθρωπος vs αυτό το άτομο. Just like you could say this person or this individual. If there is another correct translation using the latter expression or similar in Greek, then αυτό το can be used without a problem. Forum moderators don't know what the list of possible translations are. Please remember to copy and paste your whole answer in your comment so that we can clarify any grammatical points and explain why your answer may have been rejected. Otherwise, it'd be demanding a course contributor to go back and forth between different systems to manually figure out where data matches to provide an accurate explanation to a vague question. So in any case we need to see your whole answer here.
There are many reasons why a comment may not receive a reply; many posts fall through the cracks as we don't have tools to prevent this from happening.
No one wants to confuse anyone. We are passionate about helping people learn and, I'm sure I'm not the only one feeling this way, really excited to see people learning Greek specifically, as it is not a 'popular' language. We are users just like you, volunteering for hours every day answering questions, explaining/creating the material. We don't expect you to understand the inner workings of Duo (lots of them, we don't either) but we have some standards for forum discourse (link also at the bottom of this page ;)