This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Lone Guardian side quest.
Time frame: From Majin Buu Reborn - Episode 8: Finale to No time limit
Reward: 140,435 EXP, 15x D Medal, Adventure Medal
Where to start: West Area, as Trunks.
To start this mission, you need to go to Capsule Corp. Talk to Bulma - she will give you a new substory.
Your task is to find Android 17.
Defeat him to get the required items. This will complete the mission.