Osaatko käyttää duolingoa. Should be correct also.
Yes. Why is sinä important in this sentence?
It's not, it's just a missing correct answer. It's beta, it will be fixed sooner or later.
Reported again on 18 August 2020.
Again, specify you or you plural please
Either is fine if you start from the English here. So either should be accepted -- report using the flag if it isn't, and it should get added sooner or later.
Osaatko käyttää Duolingoa? needs to be added as a correct answer
29 August and they still want sinä included.