Hi,
uhm... it's tricky because the action is very simple, it simply throw a fail event because the Unity Ad API tells the action it is so...
I would enquire with Unity by writing a bug report, as well as asking on Unity forum for possible tips on that. It could be your ads setup, or something in the way you have published the app on android, these things are tricky sometimes.
but as far as the action itself, there is not much room for a potential bug inside it as is. have you properly initialized the ads system, things like that?
use UnityAdsSetDebugLevel to output more infos, maybe it will tell you something. you need that on android, not in Unity editor for it to be giving useful infos.
Bye,
Jean