A download access code should look like this example:
12ABC-DEFG3-HI4JK-5L67M-8NO99
It might be that you get a special code from a reseller, a kind of “voucher” that has to be especially activated (not unusual with software license) or can be applied in the Steinberg shop at checkout, but tbh I don’t know, and I haven’t found anything in the Steinberg Help Center.
Maybe someone else here knows, or you could ask the retailer, usually they should provide some instructions on how to deal with their codes.