Rewarded ads won't show for ios

I made a mobile game for android, first with interstitial ads and later on with rewarded ads. This all worked fine but when I switched the target platform to IOS, it gave an error. The interstitial ads on IOS works fine but when I try to view an rewarded ad on IOS it gives me this error:

Error loading Ad Unit Rewarded_iOS: INVALID_ARGUMENT - Placement Rewarded_iOS does not exist for gameId: xxxxxxx