Do you need the base game for Elden Ring: Shadow of the Erdtree?
Yes. Elden Ring: Shadow of the Erdtree is extra content and only works if Elden Ring 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 Elden Ring. Redeeming it on another account or console makes the content unusable, and that cannot be reversed.