Ukrainian colleagues of mine were a bit irritated by some expressions when they were in Poland, because often words are the same but mean different things. "Czaszka" means "skull" in Polish :-D
There seems to be an error in the app recognizing "Here's your cup" should not be marked as incorrect. It has the same meaning as "Here is your cup." Grammatically they are/they're both the same.
Duo does concatenations automatically with pronouns (e.g. He is -> He's), so we don't have to add that choice manually, but for "here" it's not automatic (turns out). Added, thanks!