MaterialDatePicker requires a value for the attribute to be set in your app theme. You can either set the attribute in your theme or update your theme to inherit from Theme.MaterialComponents (or a descendant). code example

Example: The style on this component requires your app theme to be Theme.MaterialComponents

<style name="AppTheme" parent="Theme.MaterialComponents.Light.NoActionBar.Bridge">
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorAccent</item>
        <item name="colorAccent">@color/colorPrimaryDark</item>
</style>