Substories are the side quests of DBZ Kakarot and for right now, they are highly missable, as they are only available for a limited window. Once you progress the story enough, then the substory will disappear and there’s no way to go back to try and do the mission again, unless you reload a previous save. However, on March 20, 2020, the developers added a way for you to complete any Substories you missed once you complete the main story.