Have you tried a different internet browser.
When clicking on the email link, see if at the end of the url address bar there is a NOW voucher code embedded on the link.
If there is a code it will either start with NOW or SMPT and be 12 or 14 letters / numbers long in length.
Should there be a code, copy it down and apply the voucher code next to the one month Sports Membership under My Account > My Membership on here.
Also i am assuming you are not on a current monthly Sports offer and have more than 30 days remaining or have a Day Sports Pass on your account.
Because i am not familiar with how the TalkTalk discount code is generated and if the actual Voucher code is shown on the email and not a link to add the code to your account.
Then if the voucher code is clearly visible on the email from TalkTalk and you can't manually add the code to your NOW account.
Then maybe try NOW live chat.
If you click on the green chat online button from this link page below you should get through to a NOW web chat Advisor.