add an object on the detected plane

[openmotion]

hello
how can I add an object in the center of the detected plane
in RectangleNode.swift i tried for example to add a simple sphere

let sphereGeometry = SCNSphere(radius: 20)
let sphereNode = SCNNode(geometry: sphereGeometry)
self.addChildNode(sphereNode)
sphereNode.position = position

but the sphere doesn't appears ...

thanks for your help