"Burger" should be accepted as a valid English translation.
Why is tasty not accepted on this one?
If you wrote "The hamburger is tasty" it should accept it. If not it's an error on Duolingo's part. If you wrote "burger" I also believe that this is an error, though it MAY be a different word.
'Burger', 'Tasty', etc will never be accepted unless you hit the 'my answer should be accepted' button