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Original error prompt: app is not indexable by Google search; consider adding at least one Activity with an ACTION-VIEW intent filter. See issue explanation for more details.

On SDK version 23 and higher, your application data will be automatically backed up and restored when the application is installed. Consider adding the attribute ‘Android: fullbackupcontent’ to specify the ‘@ XML’ resource, which configures the files to be backed up

More information: https://developer.android.com/training/backup/autosyncapi.html

Official explanation:

To enable Google to crawl your app content and allow users to enter your app from search results, you must add intent filters for the relevant activities in your app manifest. These intent filters allow deep linking to the content in any of your activities. For example, The user might click on a deep link to view a page within a shopping app that describes a product offering that the user is searching for.
to enable Google to capture your application content and allow users to enter your application from search results, you must add intent filters for related activities in the manifest. They allow deep links to content in any of your activities. For example, a user can click on a deep link to view a page in a shopping app that describes the product the user is searching for

About deep link: https://developer.android.com/training/app-links/deep-linking

solutions

1. Follow the prompts to add an intent filter 2

Add in the intent filter of at least one activity

<action android:name="android.intent.action.VIEW" />