"The concert is in the middle of the park."
Translation:Aia ka ʻaha mele i waena o ka pāka.
Yes, you're right e Makalika (5 months ago!). These words are called "locatives" because they indicate location, and they never take "ka" when used to indicate the location of something or someone.
Just for fun, I used "i waenakonu o" which is a much better way of saying "in the middle of" someplace, but--of course--it was zapped as incorrect, probably because it's never been taught.