Android AdRequest returns only onAdFailedToLoad AdRequest.ERROR_CODE_NO_FILL

There doesn't appear to be anything wrong with your code. I think you're receiving a genuine "no fill" response.

Ads that are deemed as child- and family-friendly are a subset of the broader ads pool. If you show ads today, after opting-in to the Designed for Families program, you might notice fewer filled impressions and lower revenue. We're working on increasing the pool of ads that are eligible to serve to DFF apps.

It is not a bug. Just a few ads has characteristics "for children 5 years" or "for children 8 years". Remove your app from "Design for families" category for showing ads or wait until there will be advertising with similar characteristics.