Some mementos ask for a specific food to be served a number of times in Game of Thorns. You can view the available foods by checking the Upgrades section after entering an event. If a food is not available to be upgraded, it will not be served.

For example: Serve Forbidden Sundae in Game of Thorns event 50 Times.

This food, while available in the Greasy Spoon Diner, is not available in the DinerTown Inn. You would have to wait until the Greasy Spoon Diner is playable for Game of Thorns to complete this memento.