This guide will explain how to have the building system work with different ground types, so for example if your level uses static meshes or blueprints as the floor instead of landscapes then you might find this guide useful.
Go to the BP_PlayerBuildingComponent and then go to the class defaults, in the class defaults search for Floors like the image below.
Floor Static Meshes – Any static meshes you add here the system will consider the ground and will allow placement for things like foundation and furniture.
Floor Actor Classes – Any blueprints classes you put here the system will consider the ground and will allow placement for things like foundation and furniture.
If after adding your levels floor static meshes or blueprint here the building system still wont allow placement then you may have something else in your level blocking the building placement.
If thats the case then check out the guide linked below, as it explains how to enable the debug mode which will tell you which actor is blocking building placement.