Remember that it is possible for you to have 2 or more side quests activated at the same time so it is not a problem if you are unable to complete one of them as you can continue it anytime you want. You can open your quest log by going to the menu and clicking on it.
This Dragon Quest XI Side Quests Guide will tell you how to do all of the Dragon Quest XI Side Quests as well as where they are found in a step-by-step manner Dragon Quest XI Side Quests There are tons and tons of side missions for you to do when playing Dragon Quest XI and they tend to add a little more depth to the game.