Because, although the two versions carry the same meaning, they are grammatically different. '...są smaczne' is the form 'to be' + adjective, whereas your form is the verb 'to taste' + adverb. Your sentence would be 'Te kanapki smakują dobrze' (Note that, in Polish we have verb+adverb, not the English verb+adjective)
Nicodemus is quite right; but recently, after long discussions, we've decided to become more liberal in terms of accepted translations of "smaczny" and "taste good" is actually among them now.
Generally though, we prefer to keep rather closer to the grammatical structure of the original sentence.
It depends on the platform you use to connect to duolingo. On PC, Microsoft Windows it's enough to install additional keyboard: „Polski Programisty” - it's the same layout as American keyboard but when you press right alt + z/x/c/a/s/e/n/o/l you get special Polish characters (aha, left ctrl + left alt does the same as right alt).