Element LinearLayout is not allowed in CardView

So, a collection of suggestions, just to be sure:

• restart AS
• invalidating Caches & restart
• Check if the right dependencies are added
• clean & rebuild of project
• try same code in another file/project
• trying to find a backup of your project
• maybe computer reboot

Check if the Gradle dependency is added properly for the CardView.

In build.gradle

implementation 'com.android.support:cardview-v7:28.0.0'

And then in the XML

<androidx.cardview.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:background="#fff"
        app:cardCornerRadius="2dp"
        android:elevation="2dp">

    <TextView android:layout_width="match_parent"
           android:layout_height="wrap_content" android:id="@+id/textView" />

</androidx.cardview.widget.CardView>

Just replace android.support.v7.widget.CardView with androidx.cardview.widget.CardView