The Greek letter is "η. We call it ήτα which in English is ee or eeta.
See here for more information. How to get the Greek keyboard and other useful links:
THE GREEK ALPHABET
HOW TO GET THE GREEK KEYBOARD These links will not only show you how to get the Greek keyboard but also how to find the Greek letters on it, how to add accents etc.
And here is another to help you navigate Duolingo
FAQ - General Questions, Bugs & Reports https://www.duolingo.com/comment/23799672
If you have questions just ask.