The answer is already on this page but here you go anyway.
These links will not only show you how to get the Greek keyboard but also how to find the Greek letters on it.
It will also help you learn the alphabet and where to find other useful links.
And here is another to help you navigate Duolingo
FAQ - General Questions, Bugs & Reports
Some simple rules to get you started:
Always read the comments before posting.
Read the Tips & notes right below the list of lessons on your Home page
Read the drop down hints. Pass your cursor over a word and a list of words will appear.
It's a great service otherwise, many thanks for everything you provide. But I still maintain that including the Greek characters could make your excellent program even better :)
What is included in each course is decided by Duolingo. The moderators are here to assist with language related issues, but of course, we try to help learners in every area. If you read some of the other comments on this page you'll find some on how to access the Greek keyboard on your device...pc, Android, iOS. This is much easier than copy/paste. There are also prepared sites to help you learn the Greek alphabet, where to find the letters on the keyboard etc.
I'm supposed to write kaka in Greek letters, but my keyboard doesn't have Greek letter keys obviously. Therefore I cannot proceed with the lesson
Obviously you should install a Greek keyboard. If you look at the top of the page, you will see a comment that explains how.