Sorry to hear you are having issues getting your phone unlocked! When you purchased this phone from Samsung, were you purchasing it under the impression that it was unlocked? If so, you may want to see about returning the phone and seeing if you can get an unlocked phone.
If this is not an option, in order to unlock your phone, you must have or have had an eligible T-Mobile USA account as well as some additional requirements in order for us to be able to unlock your phone.
You're not the first that has run into this problem. I think you'll find this thread helpful in getting your S8+ unlocked: Galaxy S8 Plus - Unable to unlock after being "white-listed" . Please let me know if you have any other questions!
Yes, my problem was fixed by contacting T- mobile on Twitter and they knew
all about how samsung does not automatically give the imei info to t-
mobile so it cannot be unlocked. Thanks!