Redeem free 3 months apple tv UK

I am trying to redeem 3 months of free apple tv offered for play station, and I get error, on PS4 - after confirmation "i aknowledge that if i download or use this in app purchase within fourteen day of clicking buy, i will no longer be eligible to cancel this purchase", when i press "buy" it sends me to authentication required - "continue on your computer phone or tab where you will be redirected to authenticate, you agree for future purchaes willl not require bank authentication. i press "continue" and then i get PURCHASE OR RENTAL NOT COMPLETE. THERE WERE ERRORS COMPLETING THIS PURCHASE OR RENTAL. WAIT A MOMENT, THEN TRY AGAIN. what can i do?