How to unlock the city
To get started, you need to start your CAREER by creating your own reader and going through the introductory segments. At some point you should reach a menu where you have options like playing the next game and more. If you scroll down in that menu, you should see an option to go to The City, but you still won't be able to enter quite.
This will bring you to a new area which is full of four blacktop courts that you can play on. You cannot enter The City until you have entered an Affiliate which requires you to reach a certain MYREP level in the game. You will start at Rookie I, you will need to reach Pro I to be noticed and offered. through an affiliation and thus enter The City itself.
After you start at Rookie I you need to increase your rank by a few levels, with Rookie II and Rookie III before you get to Pro I. The only way to level up here is to participate in the blacktop games which you will have to go up and interact with open locations to enter. There are three lands which are 3v3, while the fourth is divided into two 1v1 games.
Winning your games will definitely give you more MYREP points to improve your ranking, but you will find that it can be difficult to consistently win depending on your teammates and opponents here. If you still do well in a loss, you will get points, but it is much less than if you win. Really, your best bet is to keep grinding it until you get some good matches to win and get the points that way.
Once you hit Pro I, the game will tell you that you can choose an affiliation and access The City right now for the first time in NBA 2K21 Next Generation.