Cook training is one of the longest in the Res F; a qualified cook has to complete a summer of training (around 3 months), followed by a formal on the job training of hundreds of hours, hands on. This can be challenging to complete.
Before they are trained to conduct full food services operations (that is, plan menus and order fresh food) there are two additional, long, career courses required. So while on the line you may well see Pte and Cpl cooks, the planning in support requires a 6A qualified cook (almost always a Sgt, occasionally a MCpl).
In the Army Reserve, normally cooks are only found within the Svc Bn. You unit would have to coordinate training schedules with the Svc Bn. As well, the Svc Bn isn't given money to buy food to support your exercises; that's a cost your unit budget for the exercise would have to pay.
It's not impossible to get cooks out on an exercise, but it takes a good deal of planning and coordination. Meal planning, vehicle rentals (as the units don't have integral refrigerated storage), food ordering... there's a lot to do.
