In most cases, this is because you might have typed the registration code instead of copying and pasting it. Highlight the registration code in your order email. Then right-click and go to Copy. Then on the registration page, click inside the registration code field and then right-click and go to Paste. Make sure there is no space at the end of your registration code.
If you still receive the same message after doing the above steps, it may be that the code itself may be invalid. If you encounter this, please submit a support ticket through Zendesk and one of our technical support staff will respond promptly. Keep in mind that PhoneSheriff is only for parents to use on devices that they own and have proper consent to monitor.