My class:

global class ApprovalRecall
{
    webservice static void recallApproval(Id recId)   
    {       
        ID piwiID = [SELECT Id, ProcessInstanceId, ProcessInstance.TargetObjectId FROM ProcessInstanceWorkitem WHERE ProcessInstance.TargetObjectId =: recId].ID;
        Approval.ProcessWorkitemRequest req = new Approval.ProcessWorkitemRequest();
        req.setAction('Removed');       
        req.setWorkitemId(piwiID );
  
        Approval.process(req,false);
    }
}

Test class:

@isTest
private class ERS_Test_ApprovalRecall
{
static testMethod void testTrigger()
{
  Deals__c deal = new Deals__c();
  insert deal;
       
        // Reject the submitted request, providing a comment.
        Approval.ProcessWorkitemRequest testRej = new Approval.ProcessWorkitemRequest();
        testRej.setAction  ('Removed');
        testRej.setWorkitemId(deal.id);
   
        Test.startTest();       
            // Process the rejection
            Approval.ProcessResult testRejResult =  Approval.process(testRej);
        Test.stopTest();
       
        // Verify the rejection results
        System.assert(testRejResult.isSuccess(), 'Rejections that include comments should be permitted');
        System.assertEquals('Rejected', testRejResult.getInstanceStatus(),
          'Rejections that include comments should be successful and instance status should be Rejected');
  
}
}

Error:
System.DmlException: Process failed. First exception on row 0; first error: INVALID_CROSS_REFERENCE_KEY, invalid cross reference id: []

Can anyone help me with this?