How to Redeem Locker Codes
Open up the game and go to the MyTEAM menu, then go to the page Extras and on the top left there will be a menu called Locker Codes. This is where you can enter all of the codes. Note that in NBA 2K21 most codes expired after one week, it’s still not clear if this is the case in this years 2K as well but if so an indicator whether the code is still available will be added later on.
TL;DR: MyTEAM → Extras → Locker Codes