When using the building system you may come across issues in your level where you cant place build parts this is almost always due to something blocking its collision, this can be large collision boxes, water plains that are hidden under the ground or even small things like rocks that have collision enabled.
The easiest way to know what is blocking the build parts placement is to go to the BP_PlayerBuildingComponent and in the Class Defaults find and enable (Tick on) Debug Placement Collision.
With this setting on when you are placing the build part in game it will print the name of the blocking actor to the screen so you can see exactly what actor is blocking the placement of your build part.
Once you know what actor is the cause you can change its collision settings to ignore Build Part in its collision settings.
This process will depend on the actors type and if it has multiple component thats are blocking the collision. The best way to fix this is go to open the blueprint that is causing the issue and go though each component and If it has collision settings (not all components do) make sure its set to ignore Build Part.
If you dont have the tick boxes in your blueprint you can change the Collision Presets to Custom and this will display them.
Remember that there may be multiple actors in the level blocking the build part placement so you may have to repeat this process for each of those actors before being able to place.