RainaElf You'll need to contact EA customer support. Take a photo showing the disc, its case, and the product code (clearly readable); you can upload it when you submit the help request.
https://help.ea.com/en/customer-care/
I would strongly recommend live chat rather than email. If you don't see the option, choose a slightly different category of issue.
It is possible, although unlikely, that support will simply add Ambitions to your account. It's also possible that you'll be told the pack was previously registered, in which case you can see whether they'll help you figure out what account you might have used at the time. Even if you never added the code in the EA App or Origin, you might have used it on the Sims 3 Store, for example to get the Ambitions bonus items. If you do find another account, you can have it merged into your current one and restore your ownership of the EP that way.