If you redeemed the code while you were logged into Destiny 2, you will need to switch characters or relaunch the game. The emblem itself will be found in Collections > Flair > Emblems > General. It will only appear in your collections when you next log in, so if you have redeemed it and are checking your collections using the Companion app, or some other service tied to the API, you won't see it until then.