Can you give more detail on what happens when you try to download the game? PC or Mac? I assume bought from the Fast Spring site and not Steam. If it literally won't download, see if there is a pop-up asking if you are sure (overly aggressive computer anti-virus/malware software)
If Fast Srping, are you downloading form this thread? :
https://forums.ootpdevelopments.com/...d.php?t=356350
If it is an issue with the code not being accepted it can be something where a capital i is really a lower case L. So, copy and paste it in.
But more specifics would help.