The rock-grass transition tiles on the left are transparent, they’re designed to be used on a layer above your grass tiles. You’d have to fill the transparent parts green to make them work in a single layer.
You’d also need to have more than just one terrain to use these tiles effectively, e.g. Grass / Rocks. To avoid the ambiguity between mountain tops and valley bottoms, you might even want to define something like Grass / ValleyRocks / MountainRocks, with a couple of the all-rock tiles used for one and a couple for the other (you can fully randomize them after getting the base terrains down).