I have this issue as well, however it looks like Landmass is referencing those assets itself? So Engine content prevents Engine content from building.
I disabled ExcludeEditorContentFromBuild but the issue still persists.
LogCook: Error: Content is missing from cook. Source package referenced an object in target package but the target package was marked NeverCook or is not cookable for the target platform.
11:18:19 Source package: /Landmass/Landscape/BlueprintBrushes/Materials/Sprite/M_BrushSprite_01
11:18:19 Target package: /Engine/EditorResources/S_Terrain
11:18:19 Referenced object: /Engine/EditorResources/S_Terrain.S_Terrain
11:18:19 LogCook: Error: Content is missing from cook. Source package referenced an object in target package but the target package was marked NeverCook or is not cookable for the target platform.
11:18:19 Source package: /Landmass/Landscape/BlueprintBrushes/Materials/Sprite/M_BrushSprite_01
11:18:19 Target package: /Engine/EditorResources/S_Terrain
11:18:19 Referenced object: /Engine/EditorResources/S_Terrain
11:18:19 LogCook: Error: Content is missing from cook. Source package referenced an object in target package but the target package was marked NeverCook or is not cookable for the target platform.
11:18:19 Source package: /Landmass/Landscape/BlueprintBrushes/Materials/Internal/BrushBounds
11:18:19 Target package: /Engine/EditorMaterials/TilingAALineIntegral
11:18:19 Referenced object: /Engine/EditorMaterials/TilingAALineIntegral.TilingAALineIntegral
11:18:19 LogCook: Error: Content is missing from cook. Source package referenced an object in target package but the target package was marked NeverCook or is not cookable for the target platform.
11:18:19 Source package: /Landmass/Landscape/BlueprintBrushes/Materials/Internal/BrushBounds
11:18:19 Target package: /Engine/EditorMaterials/TilingAALineIntegral
11:18:19 Referenced object: /Engine/EditorMaterials/TilingAALineIntegral
11:18:19 LogCook: Error: Content is missing from cook. Source package referenced an object in target package but the target package was marked NeverCook or is not cookable for the target platform.
11:18:19 Source package: /Landmass/Landscape/BlueprintBrushes/Materials/Internal/BrushBounds
11:18:19 Target package: /Engine/EditorMaterials/TilingAALineBoxFiltered
11:18:19 Referenced object: /Engine/EditorMaterials/TilingAALineBoxFiltered.TilingAALineBoxFiltered
11:18:19 LogCook: Error: Content is missing from cook. Source package referenced an object in target package but the target package was marked NeverCook or is not cookable for the target platform.
11:18:19 Source package: /Landmass/Landscape/BlueprintBrushes/Materials/Internal/BrushBounds
11:18:19 Target package: /Engine/EditorMaterials/TilingAALineBoxFiltered
11:18:19 Referenced object: /Engine/EditorMaterials/TilingAALineBoxFiltered
11:18:19 LogCook: Error: Content is missing from cook. Source package referenced an object in target package but the target package was marked NeverCook or is not cookable for the target platform.
11:18:19 Source package: /Landmass/Landscape/BlueprintBrushes/Materials/Internal/BrushBounds
11:18:19 Target package: /Engine/EditorMaterials/TilingAAGrid
11:18:19 Referenced object: /Engine/EditorMaterials/TilingAAGrid.TilingAAGrid
11:18:19 LogCook: Error: Content is missing from cook. Source package referenced an object in target package but the target package was marked NeverCook or is not cookable for the target platform.
11:18:19 Source package: /Landmass/Landscape/BlueprintBrushes/Materials/Internal/BrushBounds
11:18:19 Target package: /Engine/EditorMaterials/TilingAAGrid
11:18:19 Referenced object: /Engine/EditorMaterials/TilingAAGrid