You can enable iAd network ONLY when your app is in an editable state. That is after uploading binary and before being approved.
If not, you can’t.
I am not sure how many developers forgot to enable iAd. But I am one. And I blatantly forgot to enable 1 app for over a year.. (I also use Admob, Mopub, MMedia, etc)
Why can’t we just flick the switch?