Do you have a BT TV subscription? And have you bought the Sports Membership directly from Now?
In that case, you will have two separate Now accounts. And with a BT TV subscription, the BT TV box will keep automatically reverting to the BT TV Now account every chance it gets, cutting you off from the Sports subscription.
But this will not happen on any non-BT TV device, so on those, you can log in to the direct Now account and this won’t be overridden. Though there you will lose access to any Now memberships you have through BT TV, while using the account with the direct Sports membership.
You should be able to go onto Now even on the BT TV box and override the BT TV membership to watch Sports, but you will temporarily lose the BT TV features of the box while doing this, (the Now channels in the EPG and any recordings made or scheduled to be made from them) and it will revert to the BT TV account first chance it gets, so it will be a bit of a struggle 😢