trouble unlocking phone

I paid for my ATT I phone 8 plus about 2 weeks ago. I then went to a new carrier bought a new phone an activated my number with them , I would like to unlock and sell my old ATT I Phone 8 plus  but I'm having difficulty unlocking. I get the email with the link  to confirm . I do confirm and get the screen that says "Thanks for confirming". then I check the status and it says processing. The the next day I get an email saying my request is denied because I didn't confirm within 24 hours.  I've tried this 4 times with the same results. Customer service was of no help as they basically walked me thru the same process with the same results. Any ideas ?