"Do you have a button?"
Translation:Har du en knapp?
So can you ask questions in swedish with inflection? "Du har en knapp?" with a rising pitch on knapp.
Seems legit to me. What did duo suggest?
And it's knapp, by the way. Knopp is a slang word for head. :)
Written like this it is more of an information giving, rather than asking for it. If you swap the first two words, though, it would be just fine.
"har du en knapp?" allegedly invalid in favour of "har ni en knapp?" but the latter just more formal (or for adressing multiple people at once) afaik. Both should be accepted as valid or "you" disambiguated imo.