The reason this error appears is that in the Add Mesh Components graph in BP_GridManager, decal components are only added if UseDecals is set to true. Since it is false in your case, there are no decal components at runtime and when you attempt to set the visibility of the decal during gameplay no decal is found, returning the error. This error will not cause any problems, but it is of course not ideal to have errors cluttering up the log. To prevent this I added the custom SetMarkerVisibility function in BP_GridManager, which includes a branch that prevents the nonexistent decal/mesh from being accessed. Try using that instead of the regular Set Visibility node. Or, if you are certain that you are not going to use decals for this purpose in your game, just set the visibility for the mesh and don’t bother doing it for the decal.