"Whose xylophone is this?"
Translation:Ποιανού είναι αυτό το ξυλόφωνο;
Neither the "π" nor the question mark are a problem. But you did change the word order. However, your sentence is correct and we will edit our incubator to accept ti. for now use the word order above so you can move on. Thank you for your information it will help us improve the course.
Yes you do use the masculine, but you've used the wrong case -in fact the only reason why it was corrected as a typo and not rejected is because ποιανοι does not exist as a word. It looks like a plural nominative but it simply does not exist. Ποιανού is the singular genitive, just like ποιου. You can review the declensions of ποιος on wiktionary here.
I wrote "ποιανος" and Duo said I had a spelling mistake (I have a hard time remembering /this one/ masculine is the same as the neuter). That's fine and all, but Duo also said the correct answer should have been "ποιανής". That doesn't seem right to me, the gender should match with the noun ("ξυλόφωνο", neuter), right?
Sorry, that's the first I've heard of this. We have the main transalation as shown above and that is what should be shown each time. But we also have alternatives in the event that someone uses another gender but they are not to be shown to the learner otherwise.
I'll try to see what's causing this and hope it can be fixed quickly.