Suggested fix (unconfirmed): Exit and relaunch the game to claim your rewards. If that fails, hold your cursor down on the claim button for a few seconds before letting go.
Suggested fix (unconfirmed): Switch the "Outfits" tab view from the gallery view to the "Outfits" view by clicking the book icon in the top-right corner. Ensure you have the necessary pose unlocked and selected for the outfit.
Day One Fixes