Turn FaceTime off in the settings. If your Apple ID is showing in the FaceTime settings - tap on it and sign out. I doubt that it is showing but just in case.
Go to Settings>iTunes and App Store>Apple ID. Tap your ID and sign out. Restart your iPad.
Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.
Go back to Settings>iTunes and App Stores>Apple ID. Sign in again. Then go to Settings>FaceTime>On. Try to activate again.