If you're getting the "The product key is either invalid or has already been registered." message when trying to register your plug-in on the xlnaudio.com website, first of all please make sure the product key is correct. If it is, then it's possible you have already registered the product key to your account.
After registering the product key you'll need to run the XLN Online Installer to download, install and authorize your plug-in:
If it's still not showing up it's possible that you've created two XLN accounts and you're logged in to the XLN Online Installer with a different account than the one you just registered your product key to. If this might be the case, click on your name/the person icon in the top right corner of the XLN Online Installer and select "Log out from the Online Installer", then log back in with the account you registered your product to (check which account you're logged in to on xlnaudio.com if you're not sure).
If you have rented your XLN plug-in through Splice and ran the XLN Online Installer under this period, and the product is listed in the XLN Online Installer but not authorized even after registering the product key, you might have registered the product key for the paid off license to another XLN account of yours.
If you realize that you've registered products to two different XLN accounts please get in touch with XLN Audio Support and we'll help you merge your accounts.