There are two types of plural in Polish. One is "masculine personal", and the other "not masculine-personal". The first one describes groups of people containing at least one male, and the second is for everything else. As "chłopcy" are definitely all male, it is masculine personal. And the right form is "wasi" (or "twoi").
So Duo doesn't actually want to have either.
If you made some error such that your answer was rejected, then the answer that Duo will show you as a correct answer will be either the "Twoi" version or the "Wasi" version. Which one is shown is probably random-ish.
It might also show the other version as "Another possible answer".