This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Yajirobe and Korin side quest.
Time frame: From Android Terror Arrives - Episode 2: Time frame to Episode 2: This is war
Reward: 79,713 EXP, 3x D Medal, Senzu Bean Seed, Fancy Necklace
Where to start: Western Forest Area, as Goku.
Speak with Yajirobe and then go fishing. Your task is to catch Scarlet Catfish and Goliath Catfish.
If you have these fish in your inventory, you can complete the mission immediately. You will skip the fishing section.