How do I make a Ceiling in a 3D scene

So I am making a 3D scene in GDevelop and I am trying to add a ceiling.

As a result, it should pervent you from jumping out of a map, and also used as detail.

Instead of that, it is invisible with only a piece of it being the opposite.
And it also sends me out of the map

Here is a screenshot:

How do I fix this and make an ACTUAL ceiling?