Should be "Kuinka voin auttaa sinua" accepted?
I also wonder... Please, if anyone can answer!
It should. You usually don't hear it being said, but if you want to place more emphasis on the question, then sure. It sounds a bit stiff, though, or like someone has e.g. died and you ask if there is anything at all you can do to help.
It was accepted!
Technically this is just "how can I help?", which is fine, but doesn't go with how pedantic duolingo is about word precision in general.