If Duo accepts this, then that it is counter to its philosophy. At this point, Duo is trying to get you to understand exactly what is what. "juicy" is an adjective, while "juice" is a noun. It can be important to know what part of speech something is (e.g. nouns get capitalized in German, but adjectives within a sentence do not.) So, Duo is normally quite picky about rewriting things so that, for example, an adjective replaces a noun. Later, when you are doing translations, you go more for meaning than literal translations. (As my piano teacher used to say, "First show me that you can play the piece exactly as written, and only then may you make changes to better convey whatever it is you want to convey.)
In English, "sap" does NOT mean juice from a fruit, but the liquid that is in trees and live plants. Yes, at some point the fruit was attached to the tree, but try drinking the sap of an orange tree and you will find out that it is not really the same. I do love the maple syrup that is made from the maple tree's sap.