This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Piccolo vs. Goku side quest.
Time frame: From Majin Buu Reborn - Episode 8: Finale to No time limit
Reward: 165,254 EXP, D Medal, Ultra Power Protein
Where to start: Kami's Lookout Area, as Piccolo.
To begin this mission, you must head to Kami's Lookout. Dende is waiting for you there - he will order you an additional story.
Your task is to find Goku and fight him. The enemy may be several levels above yours, so be prepared for this fight. Don't start the mission without a full health bar.
Win the battle to complete the mission.