I am sorry to read that you're unable to watch Sky Sports on your iPhone.
Do you get this message on your other device also?
As you have a device change available. Log into your TV Anywhere account (from a laptop/PC) on the right under My Profile select Sky Movies & Sky Sports. Remove the iPhone in question from the list, then go back onto the app and start streaming a channel. This will prompt for re-registration.
Hopefully this will then work for you. Let me know how you get on