plat2--> no document access records --> I have to update operation delete
 plat3---> no document access --> operation  new

plat2-->contains document access records --> Update operation delete
 plat3 -->contains document access records --> No Need to Update

There were custom validation error(s) encountered while saving the affected record(s). The first validation error encountered was "Apex trigger S360_JunctionplatformdocDATrigger caused an unexpected exception, contact your administrator: S360_JunctionplatformdocDATrigger: execution of BeforeDelete caused by: System.DmlException: Update failed. First exception on row 0; first error: MISSING_ARGUMENT, Id not specified in an update call: []: Class.S360_JunctionPlatformDocDetails.junctionDocDelete: line 157, column 1".

public static void junctionDocDelete(List<JunctionPlatformDoc__c> JunctionDocList){
    List<Document_Access__c> DocList=new List<Document_Access__c>();
    List<Document_Access__c> DocList1=new List<Document_Access__c>();
    set<id> documentset=new set<id>();
    set<id> platformset=new set<id>();
    set<id>pltIds=new set<id>();


    //Get Platform for Inserted records
    for(JunctionPlatformDoc__c junc:JunctionDocList){
        documentset.add(junc.Document__c);
        platformset.add(junc.Platform__c);
    }
    //for that document find the platform
    List<JunctionPlatformDoc__c> junctionList = [select Document__r.Name,Document__r.Description__c,Document__r.recordtypeId,Document__r.recordtype.name,Document__c,Platform__c,Platform__r.Name from JunctionPlatformDoc__c where Document__c in :documentset];


    //finding Platform
    map<id,id> platformDocmap=new map<id,id>();
    for(JunctionPlatformDoc__c junction :junctionList )
    {

        pltIds.add(junction.Platform__c);
        platformDocmap.put(junction.Platform__c,junction.Document__c);
    }

system.debug('####pltIds'+pltIds);
    //Query document Access

    List<Document_Access__c> documentList=[select id,Account__c,Processing__c,Updated_via__c,Operation__c,Platform__c from Document_Access__c];
    set<id>docmapId=new set<id>();
    for(Document_Access__c doc:documentList){
        docmapId.add(doc.Platform__c);
    }
    system.debug('docmapId@@@@@@@@'+docmapId);

    //Get other platforms from Junction Platform Doc for that document
    list<Junction_Platform__c> junctionPlat=[select id,Account__c,Platform__c from Junction_Platform__c where Platform__c in:pltIds];

    system.debug('###junction plat###'+junctionPlat);
    for(Junction_Platform__c junc:junctionPlat)
    {

        if(docmapId.contains(junc.Platform__c) && platformset.contains(junc.Platform__c)){
            Document_Access__c  doc=new Document_Access__c();
            doc.Account__c=junc.Account__c;
            doc.Document__c=platformDocmap.get(junc.Platform__c);
            doc.Platform__c=junc.Platform__c;
            doc.Processing__c=false;
            doc.Operation__c='New';
            doc.Junction_Platform__c=junc.id;
            doc.Updated_via__c='Junction Platform Doc';
            DocList1.add(doc);
        }

        else if(!docmapId.contains(junc.Platform__c)&& platformset.contains(junc.Platform__c) ){
            Document_Access__c  doc=new Document_Access__c();
            doc.Account__c=junc.Account__c;
            doc.Document__c=platformDocmap.get(junc.Platform__c);
            doc.Platform__c=junc.Platform__c;
            doc.Processing__c=false;
            doc.Operation__c='Delete';
            doc.Junction_Platform__c=junc.id;
            doc.Updated_via__c='Junction Platform Doc';
            DocList.add(doc);

        }else if(!docmapId.contains(junc.Platform__c)){
           Document_Access__c  doc=new Document_Access__c();
            doc.Account__c=junc.Account__c;
            doc.Platform__c=junc.Platform__c;
            doc.Processing__c=false;
            doc.Operation__c='New';
            doc.Junction_Platform__c=junc.id;
            doc.Updated_via__c='Junction Platform Doc';
            DocList.add(doc);

        }

    }
    if(!DocList.isEmpty()){
        Insert DocList;
    }

     if(!DocList1.isEmpty()){
        Update DocList1;
    }

}