切手（きって）を買（か）いたいなら、郵便局（ゆうびんきょく）に行（い）って来（き）て。
If you want to buy stamps, go to the post office. [in the case of]
Why can’t たら be used here?
I think なら might be used more for hypotheticals, but I’m not sure on that.
Regardless, with たら you’d have to put 買いたかったら instead of 買いたいなら. Did the question even let you fill out the verb as part of the answer?
It only lets you put なら in. I think I forgot that たら is a た verb with ら on the end, whereras なら is usually added to the plain form. I looked this up anyway, and you can also add it to the た form. たら is a one time or particular result, なら is a contectual thing. Not sure I completely understand that tbh, but clearly the example about going to the post office is not one time thing it is a general statement. Here’s the link to what I found…https://www.wasabi-jpn.com/japanese-grammar/how-conditionals-work/
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