I have a trigger that captures the approver ID in an approval process and updates a field with it on the Opportunity. I'm trying to write the unit test for it, but I'm running into errors. It is saying that there are required fields, but I have no idea what these fields are or how to set them. Has anyone been able to resolve something like this before?

Below is the specific error message, trigger code, and test code. Thank you so much for the help!

Trigger:

rigger TagSalesPlanner on Opportunity (after update) {

for (Opportunity opp : Trigger.new) {
Opportunity OldOpp = Trigger.oldMap.get(opp.ID);
if ((oldopp.ApprovalStatus__c != 'Approved' && opp.ApprovalStatus__c == 'Approved') || (oldopp.ApprovalStatus__c != 'Rejected' && opp.ApprovalStatus__c == 'Rejected')){

//get list of Approved Opportunities
List<Opportunity> ApprovedOpps = [SELECT ID, ProposalApprovers__c
FROM Opportunity
WHERE Id in: trigger.new];
    system.debug('* * * * Approved Opps List* * * *' + ApprovedOpps);

//get list of related Approval Processes
List<ProcessInstance> ap = [SELECT ID, TargetObjectID
                            FROM ProcessInstance
                            WHERE TargetObjectID in : ApprovedOpps];
                            system.debug('processInstance List' + ap);

List<ProcessInstanceStep> actor = [SELECT ActorID,ProcessInstanceID
                                   FROM ProcessInstanceStep
                                   WHERE (StepStatus='Approved' OR StepStatus='Rejected') AND ProcessInstanceID in: ap];
                                   system.debug('processInstanceStep List' + actor);

/*****************************************************
To get the list of proposal approvers in the opportunity record , I have created Maps and List to get the records.
**********************************************************/

Map<ID,List<ID>> pactors = new  Map<ID,List<ID>>();
 List<ID> l;
for(ProcessInstanceStep pa : actor){
        if(pactors.get(pa.ProcessInstanceID) == null){
           l = new List<ID>();
           l.add(pa.ActorID);
           pactors.put(pa.ProcessInstanceID,l);
           }
        else
            pactors.get(pa.ProcessInstanceID).add(pa.ActorID);
    
    }
  system.debug('processinstancemap '+ pactors);  
  
  
Map<ID,List<ID>> actors = new Map<ID,List<ID>>();
for(ProcessInstance pi: ap){
    actors.put(pi.TargetObjectID,pactors.get(pi.ID));
}
    system.debug('Process Instance map'+ actors);

for(Opportunity opp1 : ApprovedOpps){
    List<ID> approverIds = actors.get(opp1.ID);
    system.debug('approver Ids'+ approverIds);

opp1.ProposalApprovers__c='';
for (Integer i=0; i<approverIds.size(); i++){
    opp1.ProposalApprovers__c = opp1.ProposalApprovers__c + ','+ approverIds[i];}
    system.debug('Proposal approvers'+ opp1.ProposalApprovers__c);
}
update ApprovedOpps;
}
}
}

Unit Test

@isTest(SeeAllData = true)
private class Test_TagSalesPlanner{
    static testMethod void testTagSalesPlanner(){

//Set up the test records for Master Terms

Account account = new Account();
    account.Name='TestAccount';
    account.website='test.com';
    account.Industry='Entertainment';
       insert account;

Opportunity opportunity = new Opportunity();
    opportunity.AccountId = account.Id;
    opportunity.Name = 'Test Opportunity';
    opportunity.CloseDate = System.today();
    opportunity.StageName = 'In Negotiations';
    opportunity.Amount = 10000.00;
    opportunity.Type = 'New Client';
    opportunity.HasMasterDDForm__c = null;
    opportunity.approvalstatus__c = 'Not Submitted';
    opportunity.rate_type__c = 'CPM';
    
    System.debug('******** Pricebook Entry ID***********' + opportunity.PricebookEntryID__c);
    System.debug('******** Pricebook Entry Name__c ***********' + opportunity.workflowPricebookEntryName__c);
    
    insert opportunity;
    
    List<Opportunity> oppList1 = [SELECT id, workflowPricebookEntryName__c, PricebookEntryID__c FROM Opportunity WHERE id =:opportunity.id];
    System.debug('********* Pricebook Entry ID After Insert*****' + oppList1[0].PricebookEntryID__c);
    System.debug('********* Pricebook Entry Name After Insert*****' + oppList1[0].workflowPricebookEntryName__c);
    
ProposalFlight__c proposalflight = new ProposalFlight__c();
   proposalflight.opportunity__c = opportunity.Id;
   proposalflight.flightbudget__c = 100;
   proposalflight.flightstartdate__c = system.today();
   proposalflight.flightenddate__c = system.today()+60;
   proposalflight.approvalstatus__c = 'Not Submitted';
   proposalflight.versiontype__c = 'Master Terms';
   insert proposalflight;
   
ProcessInstance processInstance = new ProcessInstance();
    processInstance.TargetObjectID = opportunity.ID;
    processInstance.Status = 'Pending';
    insert processInstance;
    
ProcessInstanceStep piStep = new ProcessInstanceStep();
    piStep.StepStatus = 'Approved';
    piStep.ProcessInstanceID = processInstance.ID;
    piStep.ActorId = '00570000001ups6';

//cause trigger to fire    
    Test.startTest();
    insert piStep;
    Test.stopTest();
    system.debug('*********Approver IDs***********' + opportunity.ProposalApprovers__c);
    
//Validate that the Opportunity Flight Approval Status was updated
    List<Opportunity> oppList = [SELECT id, approvalstatus__c FROM Opportunity WHERE id =:opportunity.id];
    System.assertequals('Approved', oppList[0].approvalStatus__c);
   
//Validate that the Proposal Flight Approval Status was updated
    List<ProposalFlight__c> pfList = [SELECT id, approvalstatus__c FROM ProposalFlight__c WHERE id = :proposalflight.id];
    System.assertequals('Approved',pfList[0].approvalstatus__c);   
   }
   }

Error Message: