This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Bummed about Bananas side quest.
Time frame: From Android Terror Arrives - Episode 2: For the Future to Episode 3: The Training Room's Open!
Reward: 70,842 EXP, 3x D Medal, Shenron Plush
Where to start: King Kai's Planet Area, as Goku.
Move to King Kai's Planet to meet with the quest giver. Travel to another location and talk to Bulma.
After that, you need to move to Blake Bog.
Fly to the place indicated on the map, and then start collecting space bananas.
Bananas lie under the trees. Just land on the ground and look around - you won't have any problems with collecting all 10 items. Go back to the quest giver to complete the mission.