If you've just purchased the ornament you have to first head to your inventory tab, redeem the ornament and it should appear in your ornament slot for the weapon. They have it setup this way incase you wanted to refund the ornament.
@@theogpandayt5226 It's a nostalgic gun so they want to entice people to pay money for the more oldschool look ornament. Frustrating but it is what it is