Do you need the base game for EA Sports FC 27 Ultimate Team upgrade?
Yes. EA Sports FC 27 Ultimate Team upgrade is extra content and only works if EA Sports FC 27 is already on the same account and the same platform. Without the base game you can redeem the code but play nothing.
Same account, same platform
Buy the DLC for the platform where you already own EA Sports FC 27. Redeeming it on another account or console makes the content unusable, and that cannot be reversed.