You may find some help at the beginning of this link (don't worry about the rest for now) https://www.duolingo.com/comment/22040507 and you might find some assistance. Once you get through the ABCs it gets easier -- really. And finally, let us know what you find troublesome and we'll try to help.
The man's speech is very unclear. I distinctly heard θουθάλη with a voice 'th' and an 'ay' sound at the end. Now, if I had known that the word was 'buffalo' I could have spelled it correctly. But to hear it solely without the knowledge that it was a word I should have already encountered (this is a fault of other Duo languages, not just Greek) I hand't a clue.
I: Υοu can find a βουβάλι in the great green valley.
II: Look for a wild βουβάλι in the alley.
III: O βουβάλιος μου είναι καλός
IV: Ο βουβάλιος είναι ήρεμος.
Nominative: βουβάλι • βουβάλια • (PL)
Genitive: βουβαλιού • βουβαλιών • (PL)
Accusative: βουβάλι • βουβάλια • (PL)
το βου βά λι
The transliteration rules followed in this course are mentioned here https://www.duolingo.com/comment/17556409. Based on that post, I guess 'vuvali' would be the only correct option. However, as noted here https://www.duolingo.com/comment/23350148, the option to go through the course without using the Greek alphabet will be completely scrapped, so I suggest getting used to the Greek keyboard sooner rather than later! :)
Was the question to translate to or from Greek though?
As you will see in this post-https://www.duolingo.com/comment/23350148, the course no longer accepts the use of the Latin alphabet in place of the Greek. You will also see some of the main reasons that decision was taken.
From the form of the exercise above I am assuming it was a listening in which you were expected to write what you hear. That would be βουβάλι. Learning Greek with Latin letters would confine the learner to using it only within a confined area as on this site. To really learn the language one would need to know the Greek alphabet.
It is much easier than you would imagine. Most of the letters you know already. Have a look at this https://www.duolingo.com/comment/22040507. You will also see how to access the GReek keyboard and how to easily find the Greek letters on it.
You are in the alphabet section. You should read the Tips & Notes and the links below to show you how each letter is pronounced or as in this case groups of letters are pronounced...for example the "ου" sounds like the vowel in .... "shoe" or 'You" or 'boot'.
How are we supposed to know the difference between whether they are asking just for the letter, ir to spell the letter out? E.g., το γράμμα βήτα vs. Just β. I've checked books on the matter and cannot find a reason on why they do it that way. I don't like getting docked points when I know the right answer!