How to resolve
Error: Line: 68, Column: 1
System.DmlException: Process failed. First exception on row 0; first error: INVALID_CROSS_REFERENCE_KEY, invalid cross reference id: []

Code:
global class BMCRF_CustomApprovalProcess implements Schedulable
{    
   global void execute(SchedulableContext SC) 
   {
       //----Variable declaration--------
       string description = 'Do you want to send this to another staff member to complete this form?: No';
       list<BMCServiceDesk__Incident__c> incRecord = new list<BMCServiceDesk__Incident__c>();
       list<ProcessInstance> workItemList = new list<ProcessInstance>();
       list<ProcessInstanceWorkitem> AppoverName = new list<ProcessInstanceWorkitem>();
       map<id,id> finalList = new map<id,id>();
       map<id,string> userID = new map<id,string>();
       map<id,string> records = new map<id,string>();
       map<Id,string> approvalRecords = new map<id,string>();
       map<id,string> approverList = new map<id,string>();
       map<id,BMCServiceDesk__Incident__c> approvedRecordList = new map<id,BMCServiceDesk__Incident__c>();
       //--------------------------------
       
       //-----Fetching Incident records----------------------
       incRecord = [Select id,name,BMCServiceDesk__Service_Request_Title__c,BMCServiceDesk__Client_Name__c, BMCServiceDesk__incidentDescription__c from BMCServiceDesk__Incident__c where BMCServiceDesk__Service_Request_Title__c = 'New User Request Form For Workday' OR  BMCServiceDesk__Service_Request_Title__c = 'New User Request Form'];  
       
       for(BMCServiceDesk__Incident__c inc:incRecord)
       {
           if(inc.BMCServiceDesk__incidentDescription__c.contains(description))
           {
               records.put(inc.Id,inc.BMCServiceDesk__Client_Name__c);
           }
           
       }

       //----Fetching records in pending approval state---------
       workItemList = [SELECT Id,targetObjectId FROM ProcessInstance WHERE Status='Pending' AND ProcessInstance.targetObjectId IN: records.keySet()];
       AppoverName = [SELECT ProcessInstance.targetObjectId,ProcessInstanceId,OriginalActor.Name,OriginalActorId FROM ProcessInstanceWorkitem WHERE ProcessInstance.targetObjectId IN: records.keySet()];
       for(ProcessInstanceWorkitem record: AppoverName)
       {
          approverList.put(record.ProcessInstance.targetObjectId,record.OriginalActor.Name);
          userID.put(record.ProcessInstanceId,record.OriginalActorId);
       }
       
       //--------Getting records to be Approved----------
       
       for(ProcessInstance pi: workItemList)
       {
           id recID = pi.targetObjectId;
           if(approverList.get(recID).equals(records.get(recID)))
           {   
               finalList.put(pi.id,recID);
           }
       }
          system.debug('KeySet : '+finalList.keySet());
          system.debug(' Values : '+finalList.values() );
        for(id pi:finalList.keySet())
         {
           Approval.ProcessWorkitemRequest req = new Approval.ProcessWorkitemRequest();
           req.setComments('Approving request using CustomApprovalProcess');
           req.setAction('Approve');
           req.setNextApproverIds(new Id[]{userID.get(pi)});
           req.setWorkitemId(pi);
           Approval.ProcessResult result = Approval.process(req);
           system.debug('Incident Records : '+pi);
       }
       
     }
}