The Gradle build error AAPT: error: unbound prefix occurs during Android Asset Packaging Tool (AAPT2) compilation when an XML layout, menu, or vector drawable uses attribute prefixes like android: or app: without declaring the corresponding XML namespace (xmlns:android or xmlns:app) on the root tag.

Root XML Namespace Fix Example

res/layout/activity_example.xmlxml
<!-- CORRECT: All XML Namespaces declared on ROOT Layout Element -->
<LinearLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">
 
    <!-- Child elements can now safely use android: and app: attributes without AAPT errors -->
    <TextView
        android:id="@+id/tvTitle"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent" />
 
</LinearLayout>