Google is very confused about what it wants to name the back button of the action bar, and keeps changing it version-by-version. This is an exact copy of the back button vector, add it in drawables/ to use.

back_arrow_vector.xml

<vector xmlns:android="http://schemas.android.com/apk/res/android"
    android:width="24dp"
    android:height="24dp"
    android:viewportWidth="24.0"
    android:viewportHeight="24.0"
    android:autoMirrored="true"
    android:tint="?attr/colorControlNormal">
    <path
        android:pathData="M20,11L7.8,11l5.6,-5.6L12,4l-8,8l8,8l1.4,-1.4L7.8,13L20,13L20,11z"
        android:fillColor="@android:color/white"/>
</vector>