Any app in the Designed for Families program (or otherwise directed at children) is limited to serving ads through SDKs that have gone through Google's self-certification process confirming they don't collect data inappropriately from child users.
Commonly triggered when a kids' app integrates a general-purpose ad SDK that hasn't self-certified for the families program, often because the developer didn't check the certified-SDK list before integrating.
"Your app is directed at children but serves ads through an SDK that is not part of the Families Self-Certified Ads SDK Program, per Google Play's Families policy."