If you wanted to translate the English "We don't eat yours" to Italian, how would you know which gender and singular/plural article and possessive to use?
In this case, DuoLingo uses the plural masculine case - but why is that? Does this mean the implied subject is plural and masculine? Such as "We didn't eat your cats?"
Would you say "Non mangiamo la tua" if you were talking about an apple?
The masculine plural by itself is frequently used for referring to someone's parents. For example: "Sono andata a trovare i miei" - literally, "I went to visit mine (masculine plural)," but understood as "I went to visit my parents." Of course it could be any masculine plural thing (and the default translation is just "yours," not "your parents"), but this sentence sounds super weird out of context! But such is the Duo way. : )
Just for a laugh I tried 'let's not eat yours' - I know it's not exactly what you're teaching here but I've heard the present simple used that way consistently since I was old enough to talk, it probably should be accepted as there's not enough context to tell you the difference here