I don't know how many times you might have used the activation code, but there is a limit.
Have you tried using the license file ?
If you haven't saved your license file, you can try the Resend avast License
https://www.avast.com/resend-license-paid.php, you will need to enter the email that you used when purchasing the license, e.g. the one that the original email with the link was sent.
Have you linked/connected this installation to your my.avast.com account, you should be able to apply the license.
