When using landscapes, you typically would add named-layers into the stack in the material with the landscape layer sample node:
Once you add those to the material, they should show up as paintable-layers in the editor UI. Make sure you are in landscape edit mode and in the paint panel:
Here’s a good tutorial to bone up on material and landscape skills. It’s a bit dated, but the tech all still works in 5.x. If you are newer to Unreal, or materials at least, this might be a good place to start learning: https://www.youtube.com/watch?v=NAJhppttJgI