This is not a mistake, it is a "Warning" warning that your app is not a Bundle, according to Google itself:
Android App Bundle is the new official publication format for Android, which offers a more efficient way to create and launch apps. The Android App Bundle makes it easier for you to offer a great experience in a smaller app, which can generate more installations and reduce uninstallations. It’s easy to change. You don’t need to refactor the code to start taking advantage of a smaller app. After the change, you can count on the development of modular apps and the delivery of customizable features.
https://developer.android.com/platform/technology/app-bundle
As far as I know, there’s no way to make such an app with Ionic.