"Please wait here."
No, Duo is correct. The sentence is a little blunt but it has no grammatical problem. (Usually we say 稍等一下 / wait a little bit, instead of just 等, to make it softer.)
There is a difference between quoting a location as A. 狀語 (before the verb) and B. 補語 (after the verb). A is the location where the action takes place, whilst B is the location where the action exercises on. For verbs like 住, the difference is not prominent. 我在英國住 and 我住在英國 have no difference in meaning. However, consider the following example:
I write every vocabulary once. At home I write it on paper. At outside I write it on my palm.
In this example, the positions of 在家裡 and 在紙上 cannot be interchanged.
Unlike 住, 等 is a verb that does not exercise on a location. It is actually incorrect to say 等在 + Somewhere. We can only say where the "Waiting" takes place, 在 + Somewhere + 等.
I'm not Keith and don't know DL behind the scenes, but my understanding so far was that the Duolingo speakers come from the depth of a computer... That it's generated speech. Secondly, according to all I've heard people say in other comments, the speech seems to be generated with a Beijing accent, but the native Chinese speakers here can say more about that.