Knockout City launched on May 21 and was free to download and play for the first ten days. If you do your math, you'll realise that this offer ended yesterday, but now it turns out you can still play it for free to some extent.
This is what the official Twitter account for the game had to share yesterday:
"Block Party is over, but new players to Knockout City can still start brawlin' for free! If your friends are just joining us, they'll be able to play for free and level up to Street Rank 25 before purchasing the game. That's also enough game time to teach them to pass the ball."
Basically, learning the ropes is free and if you like it, you have to buy it. Sounds like a really consumer friendly setup. It's worth pointing out that the entire game is also included with Xbox Game Pass, so there's no need to buy it regardless of level if you are a subscriber.