"Please wait here."
This is (slightly) incorrect. This is a location complement but 等，like 住 放 坐 etc imply a movement or location and are an exemption. A google search will show the 在 after the 等 is more common than this example (although both are common).
As a native speaker, I can say that both are used and should be accepted.
Maybe it's a regional thing? I lived in Hangzhou for two years and the grammar used there would be much more like '请等在这里', to the point that I thought the other one was grammatically incorrect because I wasn't used to hearing it. If native speakers are OK with this grammar why isn't Duolingo?
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.
Let's stop that voting game and give some data: http://bcc.blcu.edu.cn/zh/search/1/%E7%AD%89%2Fv%E5%9C%A8s (corpus data for “等在 + location”)
According to what you are trying to advocate, it would have been acceptable to say 我等在家你 or 他等在车站车？Look at your search results. Those are either not modern Chinese or a specific use in a descriptive clause that is similar to using a participle as an adjective in English. It is not using a verb in a simple statement and not relevant to here.
Native opinions can be influential. Have you ever thought that what you say can be very confusing for learners?
It is time to think about focusing on providing constructive advice to help people.
Please don't do this oversimplication for me, ok? I'm just giving you real world usage. I don't want to be as pedantic as you are here, as if everyone knows the definition of “modern Chinese”. What you said could be useless “rules” defined by only yourself.
I doubt that you know (simplified) Chinese.
Those are either not modern Chinese …. It is not using a verb in a simple statement and not relevant to here.
They are collected from modern literature or translation of classics. It doesn't matter whether they are simple sentences like 请等一下. Simpler: http://bcc.blcu.edu.cn/zh/search/1/%E7%AD%89%2Fv%E5%9C%A8s%5B%EF%BC%8C%2B%E3%80%82%5D
Those are either not … or a specific use in a descriptive clause that is similar to using a participle as an adjective in English.
I can only hope you indentify their sentence structure after you accept those examples. And that's not a good reason to limit the Chinese sentence here.
In the end of this all discussion my answer 请等这里一下 is grammatically wrong, may I know why?
Because we do not say 等+location without a preposition like 在 between them, just like we do not say “wait the house” but “wait at the house”. I feel that 等这里/等那里 can be as special as “wait here/there”, but I am not so sure because I have only met them a few times and I would consider it colloquial and rather casual…
Also, 一下 may not be under consideration for flexible translation. We never know the criteria of Duolingo.
So, if I understood well : 请等在这里 is not correct, because the grammar says that 等 can't be applied to a location, but still often used in China ? I wonder why the gramme consider it wrong, since we wait in a location, like we write on a paper, or live in a location.
It would be more polite and correct to accept the answer 请在这里等一下, while not an entirely precise translation, this would be more polite, as you're only asking the person to wait a moment.