Boolean Error 3D Max

[Selective]

[Tut]

This boolean error seems to be more common on older versions of Max. I've not encountered it on more recent versions like 2021 and 2022. If you're unable to upgrade to recent versions, try converting your mesh to editable poly or detach a small selection of polygons (the ones that you want the Boolean modifier to affect) and see if it changes anything. You can also try exporting to OBJ or FBX and import that back into Max.

PS. I've moved your thread into the Modelling section, as this is not a modding tutorial.