Prepare Reward Video returns "Loading fail" message

[AhSem]

When calling the method Admob.prepareRewardVideoAd(options), I got the response as shown above. Any idea this is the Plugin message or Admob message ?

Edit: when looking at the debug message at Xcode, i saw below message:

Error Domain=com.google.admob Code=1 "Request Error: No ad to show." UserInfo={NSLocalizedDescription=Request Error: No ad to show.}