This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Pure Pair side quest.
Time frame: From Majin Buu Reborn - Episode 8: Finale to No time limit
Reward: 241,638 EXP, 3x D Medal, Porunga Plush
Where to start: Planet Namek, as Goku.
This story begins with a conversation with the quest giver. He is waiting for you on the bank of the river, on the planet Namek.
You have to answer a few questions. If you know the answers, you can choose a wish that will come true without using the Dragon Balls.