So I probably miss something simple but | Unreal Source | Page 1

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You will need to either select on the mesh partition base sections or create a sculpt modifier. The sculpt modifier is the preferred approach. You can also select a base section by clicking on the mesh partition in the editor. The selection will automatically transfer to whatever base section was clicked on. Another option is to enable showing of base sections in the scene outliner by opening the filter dropdown -> Mesh Partition -> Show Base Sections

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You can check the documentation section about sculpting and painting here: https://dev.epicgames.com/community/learning/knowledge-base/nK7J/unreal-engine-introduction-to-mesh-terrain#sculptingandpainting

Epic Games Developer

Introduction to Mesh Terrain | Knowledge base

Introduction to the new experimental Mesh Terrain plugin and its core features.

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I'll see if we can fix the error message on this this tool though, it's very misleading. You can actually add new weight attributes directly from that tool if you scroll down in the property panel

#So I probably miss something simple but