The fuel requirement was pre-assigned (for every other model of plane in the Air Canada fleet) to the flight engineer. Who that requirement was assigned to for this one model of plane wasn't really clear in the article.
It was never explicitly said. The plane used to have a 3-person crew and had moved to a 2-person crew (missing the flight engineer). This left the check unassigned. It is mentioned in the 2nd paragraph under the Investigation header.