This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for the Uninvited Training Partner side quest.
Time frame: From Majin Buu Reborn - Episode 1: New Encounters to Episode 1: Good News
Reward: 233,941 EXP, 10x D Medal, Prime Golden Wolf Meat, Butterbur Scape, Shiny Laver
Where to start: Southeast Mountains Area, as Gohan.
Talk to Videl to start this mission.
Your only task is to defeat the robot. He is two levels higher than your hero, but you shouldn't have problems defeating him.
Win the battle to complete the mission.