This page of the Biomutant game guide describes the side quest - The Stringplonk. Here we've described everything you need to do to complete the task and earn rewards.
Starting the quest
How to start: Talk to the man at the top of Peekspot 3F.
Reward: 100 XP.
To begin the quest you must come across a villager sitting at the top of Peekspot 3F. He will give you an assignment related to the Stringplonk.
Location: Leftovers 2E.
You'll find this Stringplonk in a dilapidated building, right next to the kitchen. Arrange the notes so that they match in color.
You will find this Stringplonk in an intact building, opposite the door. Arrange the notes so that they match in color.
Location: The Makeglow Ranch.
You'll find this Stringplonk in a contamination zone, it's worth getting a radiation suit, so you can explore the area in peace.