SMS Order Dish ID Prefer is not valid for "Open Dish" and dish with 0 dollars pricing.
It is used by SMS Ordering. It is the prefix for dish short code e.g. if the SMS Order Dish ID is 20 and if there are 3 pricing (Small, Medium and Large). The dish short code will be 200, 201 and 202 respectively. This is being assigned automatically by the system but its is customisable. Refer below screen shot on where to modify the "Order Item ID Prefix". 

Go to Setting>EMenu>select an item menu to edited>under Advance.