If you are a developer enrolled in the Apple Developer program and haven’t updated your device from one of the iOS 7 betas to the public release or the GM version, you may be in a bit of trouble. Several reports have emerged on the Internet that users running the beta builds of iOS 7 are getting activation errors. This is because the beta was set to expire on October 6, based on your time zone.
To fix the error, your best bet would be to update your device to iOS 7.0.2. But, make sure you are running iTunes 11.1. In case you are running an older beta, simply open iTunes, click on iTunes in the menu bar and select “Check for updates.” This is important, as iTunes 11.1 adds support for iOS 7.
Afterwards, just download the IPSW files for your device (the IPSW links can be found at the iPhone Hacks link below) and run iTunes. Hold down Shift button on Windows or options key on a Mac and click on the “Check for Updates” button. It will ask you to browse the downloaded IPSW file. Just wait until the update finishes and hopefully, your device will be updated to iOS 7.0.2.
You can check out the detailed guide in the source link that mentions step-by-step instructions on how to get rid of this activation error.