The Terrain system uses dedicated Unity engine resources to function correctly. Unity includes these resources in your application at build time when you have at least one Terrain instance in a scene that’s part of your build profileA set of customizable configuration settings to use when creating a build for your target platform. More info
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If your application only creates Terrain instances at runtime, make sure you include at least one Terrain component as a placeholder in a scene listed by your build profile. Unity detects the placeholder and includes the required resources at build time.
Tip: If you’re not using the placeholder in your application, you can hide it. Either deactivate the GameObject that contains the Terrain component or put this GameObject in a scene that your application doesn’t load.