@Manuel Reyes, Thanks for posting in Q&A.
From your description, I know you got the error when uploaded an in-house development of android app.
To upload a private Android app into Intune, there are two ways.
1.Directly upload in the Microsoft Intune admin center.
2.Uploading In the Google Play Developer Console which you need to have a Google developer account.
And for the error message "The package name is used by another application", it means the package name of your app is already in use by another app in the Google Play Store, and you cannot use it. You can ask your app developer to try using a different package name for your app and try resubmitting.
Moreover, you can try the method 2 if you have a Google developer account.
Hope above information can help.
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