Heads up/ popup push notification not showing in android kotlin

Question

This is my NotificationMessagingService I implemented all code but the pop-up notification is not showing. also please give me idea how I show the notification image that comes from backend

class NotificationMessagingService : FirebaseMessagingService() {
    override fun onMessageReceived(remoteMessage: RemoteMessage) {
        super.onMessageReceived(remoteMessage)
        if (remoteMessage.data.isNotEmpty()) {
        }
        val title: String = remoteMessage.notification!!.title!!
        val body: String = remoteMessage.notification!!.body!!
        sendNotification(title, body)
    }

    override fun onNewToken(token: String) {
        Log.d(TAG, "onNewToken: $token")
    }

    private fun sendNotification(title: String, body: String) {
        val pendingIntent = PendingIntent.getActivity(
            this, 0 /* Request code */, intent,
            PendingIntent.FLAG_ONE_SHOT or PendingIntent.FLAG_IMMUTABLE
        )
        val channelId = "channelId"
        val defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION)
        val notificationBuilder: NotificationCompat.Builder =
            NotificationCompat.Builder(this, channelId)
                .setSmallIcon(R.drawable.turno_logo_small)
                .setContentTitle(title)
                .setContentText(body)
                .setAutoCancel(true)
                .setSound(defaultSoundUri)
                .setContentIntent(pendingIntent)
                .setPriority(NotificationCompat.PRIORITY_HIGH)
                .setDefaults(Notification.DEFAULT_ALL)
        val notificationManager =
            getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            val channel = NotificationChannel(
                channelId,
                "Firebase Notifications Channel",
                NotificationManager.IMPORTANCE_HIGH
            )
            channel.setShowBadge(true)
            channel.lockscreenVisibility = Notification.VISIBILITY_PUBLIC
            notificationManager.createNotificationChannel(channel)
        }
        notificationManager.notify(0 /* ID of notification */, notificationBuilder.build())
    }
}