If you receive a message from Microsoft that your app is non-compliant, it might be because the age rating has not been filled in. This is mandatory and must be done before 30 September, otherwise you might risk that your app will be unpublished.
To bring your app into compliance, just log in to the Dev Center, click Update to edit Age Ratings and take the brief age rating questionnaire. Learn more about the process at https://msdn.microsoft.com/en-us/windows/uwp/publish/age-ratings
Feel free to contact Customer Care if you have any questions: firstname.lastname@example.org