Apple’s offer is for new subscribers. Previous subscribers that have had an extended free trial before, would not qualify for more of the same offers. You have already tried it.
Read here how to redeem: Apple TV+ offer - Apple
Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple
Read here if you have trouble redeeming: If you can't redeem your promotion for an Apple subscription - Apple Support
↳ Step 5 says to contact Apple if the prior steps didn’t bring a solution. They can assist.
$12.99 a month.
That seems to be the listed price for Canada and Australia. Are you in one of these countries?