Take a look at our documentation on how to create a landscape material and paint the material onto your landscape. It will give you a step by step approach on how to set up this workflow. There are multiple ways to set up your material, and it is all based on what textures you wish to blend with one another so that when painting between different layers, they blend correctly.
Let me know if you are still having trouble painting on your landscape. Be sure to apply the material to the Landscape within the details panel of your selected landscape.
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In short you will need to set up a line trace to detect where the object has collided, then spawn a particle system at that location.
Landscape painting is covered in detail in the documentation. You have to create a landscape material first before you can start painting. Your material must include a Landscape coordinates node and blend nodes. Whatever textures you plan on painting with need to be included here. These textures will appear in the paint tool after the material has been attached to the landscape. Once you reach this stage you can paint.