Kamu is singular 'you'
Kalian is collective 'you' (basically 'you all')
What about "Aku", why are there two words for I ? Oh, sorry, it is informal and "Saya" is formal.
Actually, I meant to originally ask about "Anda" which is also singular you, but it is formal, while "Kamu" is informal.
https://www.duolingo.com/skill/id/Basics-2/tips-and-notes You may need to click on Basics-2 skill and click on the lightbulb for the Tips and notes on the web version of Duolingo.
I think the hardest will be differentiating "Kami" we from "Kamu" you.
When i was learning indo in school i had trouble between kami and kita, rather than with kamu. Since kami is we in "just us, nor you" we thought of ka-mi as in me, so its personal. While kita is "all of us" we would use ki-tagether. Alternatively, kamu ends in a u so its YOU. I hope this helps!
Duolingo often accepts “you all” for other courses. You could try reporting it as an alternative if it was not accepted for you. It is not the standard form in English which is simply “you” for both singular and plural, so specify if it is used in your area and where that is.