Hi, to make texture painting work, you need to take a few more steps after setting up your material.
Your material setup looks good—now you need to add the layers.
Click the plus icon to “Create Layer”, and rename each layer to match the names you used in the Material Editor. For your material, Layer 1 should be Grass and Layer 2 should be Rock.
Next, you’ll see another plus icon on the right labeled “Create Layer Info”. This stores the painting data on your landscape. Click it and choose Weight-Blended Layer. Then, select a folder where the file should be saved—and you’re done! 