Refer: https://support.google.com/admob/thread/3494603/admob-error-codes-logs?hl=en

Why is  it Happening : Advertisers can target specific regions, platforms and user profiles based on business relevance, which can sometimes result in lower availability of Ads for a particular region or user profile. Error code suggests that the implementation is correct, and that the Ad Request was not filled because of lack of availability of a suitable Ad at that particular instant of time when an Ad Request was sent from the app.