Is this message happening when trying to watch the Sky Sports channels on any of your devices?
If so, a couple of things I would try.
1. Logout and sign back into your devices.
2. Try a new NOW App download from your device Apps Store.
3. If you have phone internet data, then has a temporary test try your Sports Membership with Mobile internet data on your smartphone.
4. Should the above work, then try unplugging the power to your Router for around 30 minutes, then power up the Router and try your Sports Membership again on your devices from your home network.
Or are you getting an email notification from NOW?