Test mode isn't working. I checked in samples for ios and in my own app. The response is "NoFill". Moreover, there is no difference which key i use: it can be "123" like in example or my own key from developer console.
MobileAdsiOSSDK_:Failure_LoadAdNoFill_amznAdSDK-ios-220.127.116.11 metric fired: https://fls-na.amazon.com/1/action-impressions/1/OE/mobile-ads-sas/action/MobileAdsiOSSDK_:Failure_LoadAdNoFill_amznAdSDK-ios-18.104.22.168?marketplaceId=537444737&requestId=537444737&session=537444737
I see, that some developers have the same problem.