Hi. I’ve had the same problem with Now tv 4 months ago. Bought tv from John Lewis then tried to get the free offer but no joy. Samsung kept passing me to Nowtv and back as no one was willing to take responsibility for this. Eventually JL phoned to say sign up and they would refund the money.
Have just activated my 3 months through the Samsung link. Got serial number from about TV on the settings then typed in minus the last character (in my case N).
All working now