There are other kinds of buckle than belt buckles. The 'alnIl is specifically a belt buckle, not any other kind.
But if I had a belt buckle on, with a globe on it. And I said, "the buckle represents Earth". Wouldn't that be appropriate?
But you know, if a human pointed to her belt and said, The buckle represents Earth, I'd translate it for the Klingon as tera' 'oS 'alnIl. Information can be lost in a correct translation,and sometimes it needs to be inferred.
She sees you translates to Dulegh ghaH even though information is lost in the translation. I could translate it as Dulegh be' if essential information would be lost using ghaH, but I would be adding the information that she was a be' and not a be'Hom or a ghu. maghwI' ghaH mughwI''e'.
In my opinion, so long as the learner doesn't use 'alnIl to designate shoe buckles, buckles on pilgrim hats, turnbuckles, buckles of unknown context, or a bridge that buckles under my weight, they can translate it as buckle, in the context of their belt. Especially if they routinely call their belt buckle just buckle.
I rather suspect that The buckle represents Earth is not an allowed translation because janSIy hasn't buckled down and added it yet.
Then how would one say in Klingon, "The buckle represents Earth", in that context?
We don't have a word for buckle that isn't a belt buckle. You might say tera' 'oS ngaQmoHwI' The fastener represents Earth, but ngaQmoHwI' locker, fastener is a lot more general than buckle.
So perhaps the best translation for The buckle represents Earth. is tera' 'oS 'alnIl.?
Only when the buckle you mention is a belt buckle. But if it's any other kind of buckle, you can't use the word 'alnIl. This sentence in Duolingo doesn't include any such context.
And that's why The buckle represents Earth is not an allowed translation. The buckle and the belt buckle don't mean the same thing.