On this page of the Disco Elysium guide, you will find an accurate description of the transition of the Replace lost boule side task.
In the Replace lost boule task in Disco Elysium, a metal ball must be found so that Rene and Gaston can continue their pétanque game play.
You'll get the task when you try to pass the Hand/Eye Coordination test to show men how petanque should be played (Grab the ball and show them how it's done). Unfortunately, Harry won't do very well and the bullet will land in the water.
You'll find a metal Shotput Ball under a bench in an abandoned gym at the back of the bookstore, in the Doomed Commercial Area. Take it and go back to Rene and Gaston, and the task will be done.