This problem could be caused by more than one reason. The solution is not related to the combo menu configuration but rather the menu item definitons that make up the combo menu. For example, this message appears during the combo menu item sale if the item that make up the combo menu is not in stock and there is no new stock entry in an establishment that use menu item availability. Or if there is a problem with the item class, menu level class etc. definitions of another menu item that make up the same combo menu, the combo menu cannot be sold and the error message above shows up.