If you try to access the auction house in NBA 2K21 My Team mode, you will be greeted with a message telling you that it is locked. To unlock the Auction House, you actually need to complete a specific challenge in the mode's Seasonal Diaries. This guide walks you through exactly what you need to do to unlock the Auction House in NBA 2K21.
Unlock the Auction House Season Agenda
Not very clear as to where you need to look to complete this, but the agendas are currently located on the home screen in the top left corner where it shows the season. Here you will be able to see the progress of the season, the season diaries and the lifetime diaries.
There are plenty of agendas to complete, but the one you're looking for is under the Lifetime Agendas tab. In Lifetime Diaries, scroll down to MyTeam Fanatic and you'll see that if you collect 30 cards you'll unlock the Auction House. Unlocking 30 cards should be fairly easy to do and only require a few hours of gameplay.
Once you complete this challenge, the auction house logo reward will show up, letting you know you've unlocked it. You can then freely go to the auction house to bid/sell players, franchise items, consumables and more.
