How is supposed train warp to work ?
We don't understand how to warp things between floors, we keep getting the red error message.

Have a nice day! <3

you have to make station called "WarpStation" if your train stops there it will be then teleported to the other floor (factory floor -> ground floor and any other floor or surface -> factory floor)
the important part is, to build the rails in the same physical location (you can use warp accumulator as guide)

So for example if you have WarpStation on ground floor, then you need to have rails in exactly same position in factory floor to warp there.

Eleveated rails are supported as well (aka you can warp from normal to elevated and back)