Translation:When do we arrive in Paris tomorrow morning?
Just in case anyone is interested, "wann" can also mean "[at] what time" (in appropriate contexts).
I don't know of a specific grammar rule, but I think "tomorrow morning" should directly follow "when/what time". "What time tomorrow morning (do we arrive in Paris)?" Or "At what time tomorrow morning (do we arrive in Paris)?"
I also think your sentence should be accepted. I hope you reported it.
"At what time do we arrive tomorrow morning in Paris?"
This answer was not accepted and I think it's way more accurate than the suggested translation. It's very strange to write a question starting by "when" in the same sentence where there is also a "tomorrow morning". It's obvious that the question starting by "wann" refers to a specific hour.