"There is a shoe sale at the store."
Translation:He kūʻaiemi kāmaʻa ma ka hale kūʻai.
Just as a point of information, using this pattern as a complete sentence is rare in written Hawaiian and pretty much nonexistent in conversational Hawaiian, so DL should probably not be teaching it as a complete sentence. You actually can use "aia kekahi" to say the same thing, however: "Aia kekahi kūʻaiemi kāmaʻa ma ka hale kūʻai." This describes the location of the sale, is clearly a complete sentence, and is something that DL has already taught.
Most speakers of Hawaiian, however, would probably use the word "loaʻa": "Loaʻa ke kūʻaiemi kāmaʻa ma ka hale kūʻai." Note that with "loaʻa," it's common to use the definite article "ka" or "ke" rather than the indefinite article "kekahi" that we usually use in English.
"Loaʻa" is widely used in Hawaiian (and often pronounced "loʻa" in conversational Hawaiian), but--unfortunately--there's a tendency to misuse it, and it's not particularly easy to teach or learn. But it looks like DL is apparently not up to taking on this challenge and seems to have no plans to include "loaʻa" in any of its lessons. Minamina!