"Can't add script behaviour ARKitCameraExtensions."

Same script when placed in the game object didn't open the placement indicator box (as I've sen in a tutorial), so I tried to do everything again in hope this placement box would appear, so now I can't place script at all it gives me this msg "Can't add script behaviour ARKitCameraExtensions. The script needs to derive from MonoBehaviour!" It's the same script as before. Thank you.