"Where is Harajuku's train station?"
In informal sprach/everyday Japanese, 原宿駅はどこis largely used. Yet it was graded wrong by duolingo. I must say I am really confused by the inconsistencies in politeness most of the times. While still asking for the full verb at the end, they don't use a lot of onorifics that became standard language...
Your answer sounds more normal, but I think the issue is with the phrase "Harajuku's station". The speaker is trying to convey that they're looking for a station in Harajuku that is not necessarily Harajuku Station. It could also be Meiji-jingumae Station. I don't know that the English used actually conveys that well, but I think everyone would just call 原宿駅 "Harajuku Station".