If you are getting this error, then your code is correct. The issue is that AdMob does not always have an ad to return for every request. This may happen particularly if you have just registered your AdMob publisher ID, as it takes some time and multiple requests before the new ID starts returning ads.
Another reason your fill rate may be low is that you don't have AdSense backfill enabled, or you have filtered out some ads. Check your app settings to see if that is the case.
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