The source that authors and narrators get their codes from can take up to an hour to synchronize with Audible. Thus you may get the error "The code you entered is for a promotion that hasn't started yet". Write down the code or paste it in notepad or somewhere that you can refer back it. Try using the code again in an hour or two. Don't worry; the code won't be given to anyone else.
If after a few hours the code doesn't work, please try to claim a new code and see if that works.
Unfortunately, we at FreeAudiobookCodes.com can't offer support for this as we do not have access to the book's codes or the Audible system. Please do not enter a support ticket.