Object2Terrain Error

Hello I have an error with the object2Terrain script which when i click the button this happens.

Can’t add component ‘MeshCollider’ to Plane because the game object is a generated prefab and can only be modified through an AssetPostprocessor.
Object2Terrain:Object2Terrain() (at Assets/Models/Editor/Object2Terrain.js:29)

How can i fix this?

It sounds like you didn’t follow the directions about using an object in the scene, and are trying to alter assets directly.