Ionic 3 error: resource xml/network_security_config not found

To make it work for cordova-android@^8.1.0, make sure you have this configuration:

<edit-config file="AndroidManifest.xml" mode="merge" target="/manifest/application" xmlns:android="http://schemas.android.com/apk/res/android">
    <application android:networkSecurityConfig="@xml/network_security_config" />
</edit-config>
<resource-file src="resources/android/xml/network_security_config.xml" target="app/src/main/res/xml/network_security_config.xml" />

For android@^8.1.0 you need to make sure the target is correct: target="app/src/main/res/xml/network_security_config.xml"