Overview:
The following error appears when a user tries to add a new Walk-in Attendee to the Event in CRM for iPad:
VOD_EM_ATTENDEE_AFTER_INS_UPD: execution of AfterInsert caused by: System.DmlException: Insert failed. First exception on row 0; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, An Account, Contact, or User must be selected to save a record.: [] Trigger.VOD_EM_ATTENDEE_AFTER_INS_UPD: line 113, column 1 
Root Cause:
The error occurs because the formula of the existing Attendee_Selection_vod Validation Rule on the Event_Attendee_vod object is not set correctly for the Walk-in Attendees feature.
Solution:
Modify the formula of the existing Attendee_Selection_vod Validation Rule as below:
AND(LEN( Account_vod__c ) < 1,LEN( Contact_vod__c ) < 1,LEN( User_vod__c ) < 1,LEN(EM_Attendee_vod__c) < 1,LEN(EM_Event_Speaker_vod__c) < 1, ISBLANK(TEXT(Walk_In_Status_vod__c)))
Related Documentation:
CRM Help Documentation: Walk-in Attendees