If you get this error, be sure you're attempting to authorize the right product with the right serial. The beginning of a serial number shows which product it is, i.e. SN-[Product]-....
Example: RX 7 Elements begins with SN-RX7ELE.
You can see your products and their associated serial numbers on your account:
https://www.izotope.com/en/account/purchase-history.html