Hello Paul,
Yes, if this message appears, then it means that your sales number has already been registered.
You have to use the full license number that you received via email after registration to continue using the product.
If the email is not found in inbox do check in spam and junk folders.
If you still not able to find full license number, contact our sales support using this link: https://support.avg.com/support_sales_chat?l=en to get further assistance.