Still pretty new to coding so please excuse me if my questions are rudimentary :)

I'd like to keep an audit of our OpportunitySplits. For each Opportunity that has been deleted, I would like to create a record of a custom object I created called "Opportunity Split History" that retains the history of the Opportunity Split that was deleted. I wrote some code to do this but my test class is giving me the following error message: System.DmlException: Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Missing value for required field(s): U#239.14ff (Opportunity): []. Based on the error message, I'm assuming that there is a field that is missing on the Opportunity but I can't seem to figure out which field it is. I've been looking at this for hours and I have no idea what is wrong. Any help would be appreciated. 

Here is my Test Class

@isTest(seeAllData = True)
public class OpportunitySplit_Trigger_Test {

    @isTest
	public static void Test_createOpportunitySplitHistoryDelete() {
        
		// Data setup
        User u = [SELECT Id FROM User WHERE Profile.Name = 'System Administrator' AND isActive = True Limit 1];
        User u2 = [SELECT Id FROM User WHERE Profile.Name = 'Nativo Standard User' AND isActive = True Limit 1];
		
        Id acctholdingCoRT = Schema.SObjectType.Account.getRecordTypeInfosByName().get('Holding Company').getRecordTypeId();
        Id acctClientRT = Schema.SObjectType.Account.getRecordTypeInfosByName().get('Client').getRecordTypeId();
        Id acctAgencyRT = Schema.SObjectType.Account.getRecordTypeInfosByName().get('Agency').getRecordTypeId();
        
        Id oppManagedRT = Schema.SObjectType.Opportunity.getRecordTypeInfosByName().get('Managed Service').getRecordTypeId();
        
        OpportunitySplitType oppSplitType1 = [SELECT ID FROM OpportunitySplitType WHERE DeveloperName = 'Revenue' Limit 1];
        
        List <OpportunitySplit> oppSplitList = new List<OpportunitySplit>();
        
		Account acc1 = new Account();
        acc1.Name = 'Nike';
        acc1.BillingState = 'CA';
        acc1.BillingCountry = 'US';
        acc1.RecordTypeId = acctClientRt;
        acc1.Credit_Status__c = 'Approved';
        insert acc1;
        
        Account holdingco1 = new Account();
        holdingco1.Name = 'WPP';
        holdingco1.BillingState = 'NY';
        holdingco1.BillingCountry = 'US';
        holdingco1.RecordTypeId = acctHoldingCoRT;
        holdingco1.Credit_Status__c = 'Approved';
        insert holdingco1;
        
        Account agcypar = new Account();
        agcypar.Name = 'OMG';
        agcypar.BillingState = 'NY';
        agcypar.BillingCountry = 'US';
        agcypar.RecordTypeId = acctAgencyRT;
        agcypar.Credit_Status__c = 'Approved';
        agcypar.Is_Parent_Agency__c = True;
        agcypar.Holding_Company__c = holdingco1.id;
        insert agcypar;
        
        Account agcy1 = new Account();
        agcy1.Name = 'Client Direct Child';
        agcy1.BillingState = 'NY';
        agcy1.BillingCountry = 'US';
        agcy1.RecordTypeId = acctAgencyRT;
        agcy1.ParentId = agcypar.id;
        agcy1.Holding_Company__c = holdingco1.id;
        agcy1.Credit_Status__c = 'Approved';
        insert agcy1;
        
        Opportunity opp1 = new Opportunity();
        opp1.Account = acc1;
        opp1.Agency__c = agcy1.id;
        opp1.Parent_Account__c = agcypar.id;
        opp1.Holding_Company__c = holdingco1.id;
        opp1.Agency_Credit_Status__c = agcy1.Credit_Status__c;
        opp1.Advertiser_Credit_Status__c = acc1.Credit_Status__c;
        opp1.Credit_Status__c = 'Approved';
        opp1.Amount = 100000;
        opp1.LeadSource = 'Cold Email';
        opp1.CloseDate = Date.today();
        opp1.Contract_End_Date__c = system.date.today().addMonths(2);
        opp1.StageName = 'Prospecting';
        opp1.Name = 'Nike Opportunity 07-31-2020';
        opp1.Type = 'New Business';
        opp1.RecordTypeId = [SELECT Id FROM RecordType WHERE Name = 'Managed Service' AND sObjectType = 'Opportunity' LIMIT 1].id;

        insert opp1;
        
        OpportunitySplit oppSplit1 = new OpportunitySplit();
        oppSplit1.Opportunity = opp1;
        oppSplit1.SplitPercentage = 50;
        oppSplit1.SplitOwnerId = u.id;
        oppSplit1.SplitTypeId = oppSplitType1.id;
        oppSplit1.SplitNote = 'Blah';
        
        
        OpportunitySplit oppSplit2 = new OpportunitySplit();
        oppSplit2.Opportunity = opp1;
        oppSplit2.SplitPercentage = 25;
        oppSplit2.SplitTypeId = oppSplitType1.id;
        oppSplit2.SplitNote = 'Blah Blah';
        
        OpportunitySplit oppSplit3 = new OpportunitySplit();
        oppSplit3.Opportunity = opp1;
        oppSplit3.SplitPercentage = 25;
        oppSplit3.SplitOwnerId = u2.id;
        oppSplit3.SplitTypeId = oppSplitType1.id;
        oppSplit3.SplitNote = 'Blah Blah Blah';
        

        oppSplitList.add(oppSplit1);
        oppSplitList.add(oppSplit2);
        oppSplitList.add(oppSplit3);
        
        insert oppSplitList;
        
		// Run the code
		Test.startTest();
        delete oppSplit3;
        oppSplit1.SplitPercentage = 75;
        update oppSplit1;
		
        List<Opportunity_Split_History__c> oppSplitHistList = new List<Opportunity_Split_History__c>();
        Opportunity_Split_History__c oppSplitHist1 = new Opportunity_Split_History__c();
        oppSplitHist1.Previous_Split__c = 25;
        oppSplitHist1.Previous_Split_Amount__c = 25000;
        oppSplitHist1.Previous_Split_Member__c = 'Mike Crowley';
        oppSplitHistList.add(oppSplitHist1);
        insert oppSplitHistList;
		Test.stopTest();
		
        System.assertEquals(oppSplitHistList.size(), 1);

	}
    
}

Here is my Handler Class

public class OpportunitySplit_Handler {
	
    public static void createOpportunitySplitHistoryDel(List<OpportunitySplit> oldList) {
        List<Opportunity_Split_History__c> oppSplitHistoryList = new List<Opportunity_Split_History__c>();
        for (OpportunitySplit oppSplit : oldList) {
            Opportunity_Split_History__c oppSplitHistory = new Opportunity_Split_History__c();
            oppSplitHistory.Previous_Split__c = oppSplit.SplitPercentage;
            oppSplitHistory.Previous_Split_Amount__c = oppSplit.SplitAmount;
            oppSplitHistory.Previous_Split_Member__c = oppSplit.SplitOwner.Name;
            oppSplitHistory.Date__c = Date.today();
            oppSplitHistory.Opportunity__c = oppSplit.OpportunityId;
            oppSplitHistory.History_Type__c = 'Delete';
			
            oppSplitHistoryList.add(oppSplitHistory);
            
        }
        
        if(oppSplitHistoryList.size()>0){           
    }
        insert oppSplitHistoryList;
}
    public static void createOpportunitySplitHistoryNew(List<OpportunitySplit> oldList){
        
    }
}

Here is my trigger

trigger OpportunitySplit_Trigger on OpportunitySplit (after delete) {    
    
    if (Trigger.isAfter && Trigger.isDelete) {
        OpportunitySplit_Handler.createOpportunitySplitHistoryDel(Trigger.old);
    }
    
    
}