Error showing Ad Unit Rewarded_iOS: NOT_READY - Placement Rewarded_iOS is not ready

Need Help
After getting "Unity Ads initialization complete."
When I click ShowAd(),
I got this message: "Error showing Ad Unit Rewarded_iOS: NOT_READY - Placement Rewarded_iOS is not ready."
And I am sure the ads system was working a couple of weeks ago.

Now the new error is : Error loading Ad Unit Rewarded_iOS: INVALID_ARGUMENT - Placement Rewarded_iOS does not exist for gameId: "myID"

I think loading takes a lot of time.

I think you have to use Unity Ads 3.7.5

1.
Advertisement.Load("iOS_Interstitial"); // or "Android_Interstitial"
2.
if( Advertisement.IsReady() )
Advertisement.Show("iOS_Interstitial"); // or "Android_Interstitial"

Easy to use with 3.7.5, but AAAHHH... with 4.2.1 (4.x.x) without Advertisement.IsReady()...
WHY (AND WHO) DELETED THIS FUNCTION? HEH?

This is an error in the implementation documentation:
INVALID_ARGUMENT - NO: "Rewarded_iOS", Rather: "iOS_Rewarded" and "iOS_Interstitial"