If the scene picks an abstract mesh, you might get below error and you need to getRenderingMesh
from the Abstract mesh as mentioned here: HighlightLayer not working for AbstractMesh - #3 by AnweshGangula
ERROR:
Argument of type 'AbstractMesh' is not assignable to parameter of type 'Mesh'.
getRenderingMesh CODE:
hl.addMesh(someVariableWhichContainsImportedMeshOfTypeAbstractMesh.subMeshes[0].getRenderingMesh(), BABYLON.Color3.Green());