If you're in the middle of the Frieza Saga sub-stories in Dragon Ball Z Kakarot, be careful: if you have two quests for the same NPC, you'll need to reload the area for it to appear. If you've just finished the sub story One Cool Dude, teleport somewhere else and come back to the zone for the next mission with Yamcha.
Yamcha: the Man, the Myth, the Legend
This task involves a fight between Son Gohan and Yamcha. During this confrontation, the opponent will use his spiritual ball, which he can bend in all directions to cause additional damage. You should be level 30, while he will only be at level 27. You won't win a soul emblem.