Calculate normals for gltf model

Hey folk , i posted an question befor about make Perlin noise on gltf model , but there is also a problem so i will repost it , sorry

i try to calc a computedNormal to the model but there is no showing result
i find this way but cannot know what the issue in

   vec3 computedNormal = cross(displacedPositionA -, displacedPositionB -;
        computedNormal = normalize(computedNormal);

displacedPosition for noise but i need computedNormal to use it in frensel effect , any one have an idea ?
full sandbox:

up up can any one know the solve ? :face_with_head_bandage: