When I say that the “layer sample” needs to match an existing layer on your landscape, that means when you go to the landscape paint mode, you see a layer with the same name, and that it has a layer asset assigned.
In this image, the layer “PAINTABLE_LAYER” will work. The one below called “WONT WORK!!!” will not work since there has been no layer asset created. If you see a + sign, it needs a layer asset.
If you use an already existing name that was previously set up it should still work but still double check.