It seems that the tester cannot test your app properly, which violates the Windows Store Policy and will make your app get unpublished or fail to pass the certification.
Please follow and check the notes provided by Microsoft:
If your app requires login credentials, provide us with a working demo account using the Notes to Tester field.
If your app requires access to a server, the server must be functional to verify that it’s working correctly.
If your app allows a user to add a gift card balance, give us a gift card number that can be used in the testing.
And I would like to suggest you to provide a hosted video of your app’s functionality working properly in the notes for certification when you resubmit your app so that tester can see your app perform normally and know how to test it in an appropriate way.
Please refer to Notes for certification. This can really benefit your app to pass the certification.
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solved App is not testable