Thanks for coming to the Spotify Community.
In this case, it'd be best to reach out to the folks over at Google so that they can take a look. You can reach them via one of the below ways:
Please keep in mind that the offer was only for a limited time, while supplies lasted: https://www.spotify.com/legal/premium-for-family-supplemental-offer-terms/. This means that it won't be possible to receive a new one.
Hope this helps 🙂