Only weekdays that are enabled as online delivery/pickup days for the currently selected kitchen location are listed. If no days are enabled you see the notice "You do not have Delivery/Pickup Days defined in your <location> location. Define them here!" with a link to the kitchen location settings. Enable the days there, then return to the plan.

A day that was switched on in the plan before the location stopped offering it stays visible so you can switch it off cleanly.