Hi ,

System.DmlException: Upsert failed. First exception on row 0 with id a1WV00000001HBKMA2; first error: MISSING_ARGUMENT, External_ID__c not specified: []

I have my upsert opration failing. Can any one please suggest whats wrong in the below code and how help fix the code.

public with sharing class APImageUpdatesGlobal {
    public void APFilesMissingGlobal()
        {
          string ServiceResult = '';  
          System.debug('Entered!');
      
          // Create list for Available Aupairs
          List<AP_Application__c> APFiles = [Select a.Photo4__c, a.Photo3__c, a.Photo2__c, a.Photo1__c, a.Id, a.External_ID_Photo4__c, a.External_ID_Photo3__c, a.External_ID_Photo2__c, a.External_ID_Photo1__c, a.Au_Pair__c From AP_Application__c a where a.Au_Pair__r.AP_Status__c ='Available' AND a.Au_Pair__r.Record_Type_Name__c= 'Au Pair'];      
          
          string XmlString = '<FilesData>';                
          for(AP_Application__c APFile: APFiles)
            {    
              XmlString += '<APFiles>';
              XmlString += '<ApplicationId>'+APFile.Id+'</ApplicationId>';
              XmlString += '<APID>'+APFile.Au_Pair__c+'</APID>';
              XmlString += '<Path>'+APFile.Photo1__c+'</Path>';  
              XmlString += '<External_ID>'+APFile.External_ID_Photo1__c+'</External_ID>';
              XmlString += '<Inactive_Dt></Inactive_Dt>';  
              XmlString += '</APFiles>';    
              
                XmlString += '<APFiles>';
              XmlString += '<ApplicationId>'+APFile.Id+'</ApplicationId>';
              XmlString += '<APID>'+APFile.Au_Pair__c+'</APID>';
              XmlString += '<Path>'+APFile.Photo2__c+'</Path>';
              XmlString += '<External_ID>'+APFile.External_ID_Photo2__c+'</External_ID>';
              XmlString += '<Inactive_Dt></Inactive_Dt>';
              XmlString += '</APFiles>';  
              
              XmlString += '<APFiles>';
              XmlString += '<ApplicationId>'+APFile.Id+'</ApplicationId>';
              XmlString += '<APID>'+APFile.Au_Pair__c+'</APID>';
              XmlString += '<Path>'+APFile.Photo3__c+'</Path>';
              XmlString += '<External_ID>'+APFile.External_ID_Photo3__c+'</External_ID>';
              XmlString += '<Inactive_Dt></Inactive_Dt>';
              XmlString += '</APFiles>';  
              
              XmlString += '<APFiles>';
              XmlString += '<ApplicationId>'+APFile.Id+'</ApplicationId>';
              XmlString += '<APID>'+APFile.Au_Pair__c+'</APID>';
              XmlString += '<Path>'+APFile.Photo4__c+'</Path>';  
              XmlString += '<External_ID>'+APFile.External_ID_Photo4__c+'</External_ID>';
              XmlString += '<Inactive_Dt></Inactive_Dt>';                
              XmlString += '</APFiles>';            
            }
              //Close root Xml
            XmlString += '</FilesData>';         
            GAPOrg.AjaxSoap Servicestub = new GAPOrg.AjaxSoap();
            ServiceResult = Servicestub.GAPAPFilesGlobalRun(XmlString);          
            Dom.Document docx = new Dom.Document();
              docx.load(ServiceResult);
              Dom.Xmlnode root = docx.getRootElement();
              List<Files__c> upsertfiles = new List<Files__c>(); 
              for(Dom.Xmlnode child : root.getChildElements())    {             
               Files__c file = null;           
               for(Dom.Xmlnode node : child.getChildElements())      {                  
                   if(node.getName() == 'External_ID') {                    
                     string str = node.getText();               
                          if( str != ''){
                             file = new Files__c();        file = [Select f.Path__c, f.Inactive_Date__c, f.Id, f.External_ID__c, f.Contact__c, f.AP_Application__c From Files__c f where f.External_ID__c =: node.gettext() limit 1 ];
                     }
                 } 
                 else if(node.getName() == 'Inactive_Dt')
                 {
                   if(file != null) {
                     if(node.getText() != ' ')
                     {
                      String[] stringDate = node.gettext().split('/');                     
                       Integer m =  Integer.valueOf(stringDate[0]);
                       Integer d = Integer.valueOf(stringDate[1]);
                       Integer y = Integer.valueOf(stringDate[2]);  
                      file.Inactive_Date__c = date.newinstance(y,m,d);                   
                     }
                   }
                 }
               }
               if (file == null) {  
               for(Dom.Xmlnode node : child.getChildElements())
               {
                  string str = node.getName();                 
                  file = new Files__c();                     
                 if(str == 'ApplicationId')
                 {    file.AP_Application__c = node.gettext();     }
                 else if(str == 'APID')
                 {  file.Contact__c = node.gettext();    }
                 else if(str == 'Path')
                 {   file.Path__c = node.gettext();   }
                 else if(str == 'Inactive_Dt')
                {
                 if(str != ' ')
                   {
                    String[] stringDate = node.gettext().split('/');                  
                     Integer m =  Integer.valueOf(stringDate[0]);
                     Integer d = Integer.valueOf(stringDate[1]);
                     Integer y = Integer.valueOf(stringDate[2]);  
                    file.Inactive_Date__c = date.newinstance(y,m,d);
                   }
                 }
               }
           }      
          upsertfiles.add(file);   
          file =null;                    
        }         
        try {   upsert upsertfiles External_ID__c;      }
        
          catch (DmlException e) {
                System.debug(e.getMessage());
           }    
     }
}