Can you say 请问，我们在哪里可以换钱?
That would actually be the most natural construction according to Chinese Grammar Wiki: https://bit.ly/2D6UU1Z
Prepositional phrases + verb + object
There are too many errors on Duolingo Chinese, personally I have given up on it.
Yes, it's perfectly acceptable even though Duo inexplicably marks it wrong.
I agree. It is perfectly fine to a native speaker.
或者 请问，我们哪里可以还钱 不对吗
Could you say 我们那里可以换钱？
Well, for one thing, you have the wrong "na" (na3 li3 means where, while na4 li3 means there).
我们哪里可以换钱 is acceptable. But it is not a translation.
He means 在. Is it ok to not use 在 it in this sentence