The zip code is assigned to a route that does not run on that weekday, or the day is not enabled for the kitchen. Open Delivery Routes & Methods, edit the route and tick the day under Meal Prep Delivery Schedule (or Catering Delivery Schedule for catering orders). If the day is not listed there at all, enable it first under Kitchen Locations > Meal Prep fulfilment. Alternatively, add the zip a second time on another route that runs that day.