If a person purchases the Kindle Fire as a gift and forget to mark it as a gift it will be registered to buyers account. You will need to remove that buyers account before the Kindle you can register it.

• There are two ways to deregister your Kindle, through online using your computer or directly from the device

Deregister through Computer

• Head over to the Manage Your Kindle webpage.
• Select Manage Your Devices.
• Find the device you want to unregister. Select the Deregister link next to it.

Deregister directly from Kindle Fire

• Make sure your device is connected to the internet using Wi-Fi. Select the Quick Settings icon.
• Select the More icon.
• Select My Account.
• Now hit the Deregister button to finish the job.
• Click here to see how to register again on Kindle