Ensure your computer is connected to the internet during activation, as the software needs to communicate with the activation server.
Use their by entering the original email address you used during purchase.
If you are unable to fix the Pro licensing issue, consider using the FlashBack Express 5 version.
New licenses for FlashBack Pro 5 are increasingly rare as the developer, Blueberry Software, has pivoted toward newer versions and the Express model. However, you can still find official channels for purchasing or renewing: