Hi @Anonymous User
Could be because you have an existing one day or one week sports pass saved on your account ready to use (if you don't see the green apply voucher button or the buy pass button next to either the one week or month pass that could be one reason).
The other possible reason is that you are not eligible for the 6 months sports pass at £20 per month or not fortunate enough to be offered the Black Friday deal by NowTV.
I am also assuming that you was attempting to buy the 6 months £20 per month pass on the NowTV website for new customers that create a new account by following the link ?
Perhaps see if live chat can help you out.