Is the Sports Membership showing on your online account under My Membership?
If so have you tried logging out and back in again with the same account details that shows the Sports Membership on your online account.
Depending on your playback device you either click on the gear wheel icon to reach My Account or scroll down to the bottom of the NOW App home screen and click on My Account tile box.
Also have you tried all your playback devices to see if this Sports Membership is working ?
On your membership account does it show future up and coming payments for it?
Otherwise it sounds like an issue with the offer being applied. Jump onto live chat to see if they can sort it for you.
Make sure to click “chat online” within the green box, don’t hit “get help quicker” as this is a bot.
So are you still getting no joy ?
Other things that can cause this message is using a VPN, trying to watch outside the UK or your Router IP address is seen by NOW either unknown region or outside the UK where sometimes unplugging the power to the Router for around 20 minutes can fix it.
If you have Mobile data try that with a smartphone to see if there is something causing it on your Router.