Is there a specific reason why it says you can't use it (without giving ny sensitive info) ?
You could try the - Sales change order details, wrong email address given, etc. - Submit a support ticket
https://support.avast.com/en-ww/contact/sales in your case select the Billing,
payment, or license issues option - and then the I have a different billing or license issue and click Next and follow.
The payment is loosely the closest to your problem. It may ask you for an order ID, which in your case you don't have (for this intended purchase), use 00000000 as the ID and give the information given here, that should at least allow you to send your support ticket.