Did you add these lines of code?

You need to configure a top level or static method which will handle the action:

@pragma('vm:entry-point')
void notificationTapBackground(NotificationResponse notificationResponse) {
  // handle action
}

Specify this function as a parameter in the initialize method of this plugin:

await flutterLocalNotificationsPlugin.initialize(
    initializationSettings,
    onDidReceiveNotificationResponse: (NotificationResponse notificationResponse) async {
        // ...
    },
    onDidReceiveBackgroundNotificationResponse: notificationTapBackground,
);