99% of the time an error message will show because the SIM card is not inserted correctly. Please check your handset user guide to make sure it is fitted properly.
If possible, try the SIM card in another handset, this could determine if it's a SIM or handset issue. Next, try cleaning the SIM and the metal contacts with a dry cloth (the kind you use to clean sunglasses).
If you still have a problem, then please call Kogan Mobile Customer Service on 1300 056 426 (1300 0 KOGAN) and we'll do our best to help out.