I am facing same issue and after spending time found a solutons you can try below code. If your using Service
then put this code in onCreate else your using Intent Service
then put this code in onHandleIntent.
if (Build.VERSION.SDK_INT >= 26) {
String CHANNEL_ID = "my_app";
NotificationChannel channel = new NotificationChannel(CHANNEL_ID,
"MyApp", NotificationManager.IMPORTANCE_DEFAULT);
((NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE)).createNotificationChannel(channel);
Notification notification = new NotificationCompat.Builder(this, CHANNEL_ID)
.setContentTitle("")
.setContentText("").build();
startForeground(1, notification);
}