The following page of the FF7 Remake guide contains a walkthrough for Rat Problem. You can get this mission from the trader in Chapter 3 - Home Sweet Slum.
Time for one of the most classic tasks in RPGs - rats' extermination. Go to the place indicated by the trader and you will come across a bunch of rats. The fight is quite simple - use Assess Materia to examine enemies' weaknesses for Chadley's Report mission.
After defeating the group of rats, you must withdraw from the area for a while and return after a certain amount of time for the second wave of rats to appear. The best solution is to kill Wererats and go to the trader to report to him about exterminating the first wave, and then go back to kill more rats. The second wave is a little more difficult because you will be fighting Doomrats.
After the fight, return to the trader.
Reward: 5 Hi-potions.