Any layer from the "Layers" or "Ecosystem layers" is bound to give you this message. If you load a layer as a base material, then load another layer, there are 2 alpha informations: the one from base layer, and the one from top layer. Base layer cannot have alpha productrion, it must cover the entire object/terrain.  Vue is lost at this stage and asks you for help. As a rule of thumb, never use a Layer as a base material, or if you do so, reset the alpha production function.