Ιλιάδα is a feminine gender noun and the feminine definite article in singular is η and not το, which is the definite article of the neuter gender.
If you are using the Duolingo app, for more Grammar explanations I suggest checking the Tips & notes section under the lessons on the website. They are available on the desktop (not mobile) website only. Also, check the sticky notes with the resources, there are tons of information out there! :)
There's also option for add keyboard handwriting in the same place that have informed by harukoharuhara.
You need to go into your phone's keyboard settings and install Greek on your keyboard.