When the Events Management digital sign-in sheet is being utilized, there is no option to opt-in/opt-out for a meal.
The user does not have Edit Field Level Security (FLS) to the Event_Attendee_vod__c.Meal_Opt_In_vod__c field. Or, the Meal Opt-in is not enabled.
To enable Meal Opt-In, the Meal_Opt_In_vod field on the Event_Attendee_vod object is placed on the page layout.
In addition to enabling the feature, ensure that the user has FLS Edit access to the Meal_Opt_In_vod field on the Event_Attendee_vod object
CRM Help Documentation: Using the Digital Sign-in Sheet