i was looking at this page here:
https://answers.unrealengine.com/questions/62484/view.html
and apparently these “additive/subtractive” only works on other Brush Objects. So it might not work if your walls are not already brush objects
ALSO, something i noticed (and this is mentioned in that linked page) is that you need to create this from an EXISTING ATTITIVE SHAPE.
so when I just tried it out on my UE4 v4.19.2, for kicks i placed a subtractive box shape in the field, then an additive box shape, but this did not do anything
HOWEVER, if i DUPLICATED the element within the viewport, by clicking on the object and dragging on an arrow while holding down alt
, then it actually did something to the additive box
this is just something i noticed, not sure if this will be helpful for you.