Ich hab es nicht getestet:

void Update()
{
    Ray ray = Camera.main.ScreenPointToRay(input.mousePosition);
    if(Physics.Raycast(ray, out var hit) && Input.GetMouseButtonDown(1))
        {
            Vector3 blockPosition = hit.transform.position + hit.normal;
            GameObject Cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
            Cube.transform.position = blockPosition;
        }
}