The Requests you receive from Elizabeth in Persona 3 Reload have various objectives and rewards. Depending on the Request itself, you'll have a deadline to complete it, usually as a result of story-related events. "Request 68: Bring me a fruit knife" has a swift-approaching deadline, so you'll want to finish it as soon as you can. The Fruit Knife quest-item in Persona 3 Reload sounds as if it can be picked up from the Iwatodai Dorm kitchen, but that isn't the case. Instead, you'll need to speak to one of your party members to obtain it.