The following page of the FF7 Remake guide contains a walkthrough for Nuisance in the Factory side quest available in Chapter 3 - Home Sweet Slum.
Talk to another trader and you will find out about his problems with creatures (Lesser Drakes). Go to the abandoned factory and talk to Narjin to let you into the building.
Run around the factory and fight the monsters. Your goal is to kill two groups of Lesser Drakes. Enemies aren't particularly strong, but they can fly - this makes them harder to hit. Cloud's auto attack combined with e.g. Thunder Materia should suffice.
After the fight, go to the trader and complete the mission.
Reward: 500 Gil.