The Terrain system uses Unity engine resources to function correctly. Unity includes these resources in your Player build when you have at least one Terrain instance in a Scene that is part of the build. If you only create Terrain dynamically at runtime, make sure you include a Terrain component somewhere in your build as a placeholder. You can disable that Terrain instance (uncheck the box next to the Terrain name in the Inspector window), and put it in a Scene that your game or app doesn’t load.