I am Updating Some fields in a SiteContracts__c Object from Engineer_Checklist__c object , while performing this Operation getting the Above mentioned Error, Can any one suggest me where should I Specify the ID and How ?

Here is my Code:

public List<id> sitecontractids = new List<id>();
public List<Engineer_Checklist__c> ssnewList = new List<Engineer_Checklist__c>();
public List<SiteContracts__c> scnewlist = new List<SiteContracts__c>();
public List<SiteContracts__c> scList =[SELECT id, name From SiteContracts__c LIMIT 200];

for(SiteContracts__c scnewid :scList ){
         sitecontractids.add(scnewid.id);
   }

  ssnewList =[Select Id, X6L_Water_Qty__c, X9L_Water_Qty__c, X3L_Water_Mist_Qty__c, X6L_Water_Mist_Qty__c, X9L_Water_Mist_Qty__c
                                      From Engineer_Checklist__c
                                                  Where Fire_Extinguisher_Service_complete__c=true AND Site_Contract__c !=NULL AND
                                                                   Site_Contract__c IN :sitecontractids  Order By lastmodifieddate desc LIMIT 200];

for(Engineer_Checklist__c ssli:ssnewList)
{
    SiteContracts__c newsc = new SiteContracts__c();

newsc.X6L_Water__c=ssli.X6L_Water_Qty__c;
newsc.X9L_Water__c=ssli.X9L_Water_Qty__c;
newsc.X3L_Water_Mist__c=ssli.X3L_Water_Mist_Qty__c;
newsc.X6L_Water_Mist__c=ssli.X6L_Water_Mist_Qty__c;
newsc.X9L_Water_Mist__c=ssli.X9L_Water_Mist_Qty__c;

scnewlist.add(newsc);
}
update scnewlist;