When you're upgrading your phone make sure to check your IMEI number.
This error can occur if there is an issue with your IMEI number. This is the unique number (it’s a long one) which can be found behind the battery in your mobile phone. In order to upgrade your IMEI number must be identified by Vodafone as being eligible for upgrade. If it hasn't, or there is another issue with this number, then you'll see this error message. Make sure to input the correct digits when you try again.