This version of Unity is unsupported.
Method group is Obsolete

Terrain.materialType

Obsolete public MaterialType materialType;

Description

The type of the material used to render the terrain. Could be one of the built-in types or custom. See Terrain.MaterialType.

If you want to use a custom material, set this to Terrain.MaterialType.Custom, then assign a material to Terrain.materialTemplate. Terrain.MaterialType.Custom with Terrain.materialTemplate == null is identical to Terrain.MaterialType.BuiltInLegacyDiffuse.