The native way to say this should be “请上十五层来。"
Yes, that is also acceptable, which usually appears in mainland China and middle Chinese areas.
Why is the 上 in front of the floor, not behind?
上 is the verb in this case.
It doesn't mean "up"?
You can drop the 楼 in the sentence
In my experience EVERY SINGLE TRANSLATION that begins with 清 has some glitch, such that even the accepted answer is rejected when you use the keyboard. You need to use the word bank. Which is very annoying.