I am getting the following error message:

System.DmlException: Insert failed. First exception on row 0; first error: INVALID_FIELD_FOR_INSERT_UPDATE, Business Relationship may not use Relationship field : Mailing Street: [Mailing Street]

Stack Trace:

Class.TestCreateNewDistributionDestination.testUpdateCaseToCreateNew: line 24, column 1

The organization that this class is in uses person accounts, and I updated the code to address this from another poster's suggestion (creating accounts, then contacts, then merging them), however I am now running into this error.

Here's the code, any help would be much appreciated! This is my first test class attempt, and I am very stuck.

@isTest
private class TestCreateNewDistributionDestination{
    static testMethod void testUpdateCaseToCreateNew(){
        ////Query RecordType Id to Create PersonAccounts for Test////
        Id RecId = [
            SELECT r.Id, r.Name, r.DeveloperName, r.IsPersonType 
            FROM RecordType r 
            WHERE sObjectType = 'Account' AND IsPersonType=True AND DeveloperName='Individual'
        ].Id;

        system.debug('RecordId:' + RecId);
        
        ////Create PersonAccounts for Test by Creating Accounts, Then Contacts, Then Merging Them////
        
        account[] accs = new account[] {
            new account(Name='Mr. John Sandbox1'),
            new account(name='Mr. John Sandbox2')
        };

////****THIS LINE IS LINE 24 AND CAUSES THE ERROR****////

        insert accs;
        
        contact[] cons = new contact[] {
            new contact(accountid=accs[0].id,salutation='Mr.',firstname='John',lastname='Sandbox1'),
            new contact(accountid=accs[1].id,salutation='Mr.',firstname='John',lastname='Sandbox2')
        };
        insert cons;
        accs[0].recordtypeid = accs[1].recordtypeid = RecId;
        update accs;
        
        system.debug('Accounts Have Been Created');
        
        Map<String,Id> DDRTMap = new Map<String,Id>{};
        
        ////Generate a List of Disbursement Destination Record Types////
        List<RecordType> DDRTList = [
            SELECT r.DeveloperName, r.Id, r.Name
            FROM RecordType r
            WHERE sObjectType = 'Disbursement_Destination__c'];
        
        ////Put the List into a Map////
        for(RecordType RT: DDRTList){
            DDRTMap.put(RT.DeveloperName,RT.Id);
        }        

        system.debug('<<<<ABOUT TO INSERT CASES>>>>');
        
        ////Create 6 Cases for the Test; 2 of each of 3 record types; 1 insert, 1 update of each////
        Case[] Css = new Case[]{
            new Case(
                Account = accs.get(0), 
                Type = 'Wire Request', 
                RecordTypeId = DDRTMap.get('Disbursement_Destination_Wire'),
                Existing_or_New_Disbursement_Destination__c = 'Use/Create a New Disbursement Destination',
                Receiving_Account_s_Number_Case__c = '135792468',
                Receiving_Account_s_Name_Case__c = 'TEST Receiving Account',
                Payee_s_Name_Case__c = accs.get(0).name, 
                Payee_s_Street_Case__c = '1 Test Lane', 
                Payee_s_City_Case__c = 'Testville', 
                Payee_s_State_Case__c = 'NJ', 
                Payee_s_Zip_Code_Case__c = '12345',
                Receiving_Bank_s_Name_Case__c = 'TEST BANK',
                Receiving_Bank_s_ABA_ACH_Case__c = '123456789',
                Receiving_Bank_s_ABA_Wire_Case__c= '123456789',
                Receiving_Bank_s_Street_Case__c = '1 Bank Street', 
                Receiving_Bank_s_City_Case__c = 'Bankstown', 
                Receiving_Bank_s_State_Case__c = 'NJ', 
                Receiving_Bank_s_Zip_Code_Case__c = '98765'
            ),
            
            new Case(
                Account = accs.get(1), 
                Type = 'Wire Request', 
                RecordTypeId = DDRTMap.get('Disbursement_Destination_Wire'),
                Existing_or_New_Disbursement_Destination__c = 'Leave Disbursement Destination Blank',
                Receiving_Account_s_Number_Case__c = '135792468',
                Receiving_Account_s_Name_Case__c = 'TEST Receiving Account',
                Payee_s_Name_Case__c = accs.get(1).name, 
                Payee_s_Street_Case__c = '1 Test Lane', 
                Payee_s_City_Case__c = 'Testville', 
                Payee_s_State_Case__c = 'NJ', 
                Payee_s_Zip_Code_Case__c = '12345',
                Receiving_Bank_s_Name_Case__c = 'TEST BANK',
                Receiving_Bank_s_ABA_ACH_Case__c = '123456789',
                Receiving_Bank_s_ABA_Wire_Case__c= '123456789',
                Receiving_Bank_s_Street_Case__c = '1 Bank Street', 
                Receiving_Bank_s_City_Case__c = 'Bankstown', 
                Receiving_Bank_s_State_Case__c = 'NJ', 
                Receiving_Bank_s_Zip_Code_Case__c = '98765'
            ),
            new Case(
                Account = accs.get(0), 
                Type = 'Wire Request', 
                RecordTypeId = DDRTMap.get('Disbursement_Destination_ACH'),
                Existing_or_New_Disbursement_Destination__c = 'Use/Create a New Disbursement Destination',
                Receiving_Account_s_Number_Case__c = '135792468',
                Receiving_Account_s_Name_Case__c = 'TEST Receiving Account',
                Payee_s_Name_Case__c = accs.get(0).name, 
                Payee_s_Street_Case__c = '1 Test Lane', 
                Payee_s_City_Case__c = 'Testville', 
                Payee_s_State_Case__c = 'NJ', 
                Payee_s_Zip_Code_Case__c = '12345',
                Receiving_Bank_s_Name_Case__c = 'TEST BANK',
                Receiving_Bank_s_ABA_ACH_Case__c = '123456789',
                Receiving_Bank_s_ABA_Wire_Case__c= '123456789',
                Receiving_Bank_s_Street_Case__c = '1 Bank Street', 
                Receiving_Bank_s_City_Case__c = 'Bankstown', 
                Receiving_Bank_s_State_Case__c = 'NJ', 
                Receiving_Bank_s_Zip_Code_Case__c = '98765'
            ),
            
            new Case(
                Account = accs.get(1), 
                Type = 'Wire Request', 
                RecordTypeId = DDRTMap.get('Disbursement_Destination_ACH'),
                Existing_or_New_Disbursement_Destination__c = 'Leave Disbursement Destination Blank',
                Receiving_Account_s_Number_Case__c = '135792468',
                Receiving_Account_s_Name_Case__c = 'TEST Receiving Account',
                Payee_s_Name_Case__c = accs.get(1).name, 
                Payee_s_Street_Case__c = '1 Test Lane', 
                Payee_s_City_Case__c = 'Testville', 
                Payee_s_State_Case__c = 'NJ', 
                Payee_s_Zip_Code_Case__c = '12345',
                Receiving_Bank_s_Name_Case__c = 'TEST BANK',
                Receiving_Bank_s_ABA_ACH_Case__c = '123456789',
                Receiving_Bank_s_ABA_Wire_Case__c= '123456789',
                Receiving_Bank_s_Street_Case__c = '1 Bank Street', 
                Receiving_Bank_s_City_Case__c = 'Bankstown', 
                Receiving_Bank_s_State_Case__c = 'NJ', 
                Receiving_Bank_s_Zip_Code_Case__c = '98765'
            ),
            new Case(
                Account = accs.get(0), 
                Type = 'Wire Request', 
                RecordTypeId = DDRTMap.get('Disbursement_Destination_Check'),
                Existing_or_New_Disbursement_Destination__c = 'Use/Create a New Disbursement Destination',
                Receiving_Account_s_Number_Case__c = '135792468',
                Receiving_Account_s_Name_Case__c = 'TEST Receiving Account',
                Payee_s_Name_Case__c = accs.get(0).name, 
                Payee_s_Street_Case__c = '1 Test Lane', 
                Payee_s_City_Case__c = 'Testville', 
                Payee_s_State_Case__c = 'NJ', 
                Payee_s_Zip_Code_Case__c = '12345',
                Receiving_Bank_s_Name_Case__c = 'TEST BANK',
                Receiving_Bank_s_ABA_ACH_Case__c = '123456789',
                Receiving_Bank_s_ABA_Wire_Case__c= '123456789',
                Receiving_Bank_s_Street_Case__c = '1 Bank Street', 
                Receiving_Bank_s_City_Case__c = 'Bankstown', 
                Receiving_Bank_s_State_Case__c = 'NJ', 
                Receiving_Bank_s_Zip_Code_Case__c = '98765'
            ),
            
            new Case(
                Account = accs.get(1), 
                Type = 'Wire Request', 
                RecordTypeId = DDRTMap.get('Disbursement_Destination_Check'),
                Existing_or_New_Disbursement_Destination__c = 'Leave Disbursement Destination Blank',
                Receiving_Account_s_Number_Case__c = '135792468',
                Receiving_Account_s_Name_Case__c = 'TEST Receiving Account',
                Payee_s_Name_Case__c = accs.get(1).name, 
                Payee_s_Street_Case__c = '1 Test Lane', 
                Payee_s_City_Case__c = 'Testville', 
                Payee_s_State_Case__c = 'NJ', 
                Payee_s_Zip_Code_Case__c = '12345',
                Receiving_Bank_s_Name_Case__c = 'TEST BANK',
                Receiving_Bank_s_ABA_ACH_Case__c = '123456789',
                Receiving_Bank_s_ABA_Wire_Case__c= '123456789',
                Receiving_Bank_s_Street_Case__c = '1 Bank Street', 
                Receiving_Bank_s_City_Case__c = 'Bankstown', 
                Receiving_Bank_s_State_Case__c = 'NJ', 
                Receiving_Bank_s_Zip_Code_Case__c = '98765'
            )
        };
        
        system.debug('Cases Have Been Created, But Not Inserted');
        
        ////Insert the cases, update the cases needed
        test.starttest();
        insert Css;
        system.debug('Inserted Css');
        Css[1].Existing_or_New_Disbursement_Destination__c = 'Use/Create a New Disbursement Destination';
        Css[3].Existing_or_New_Disbursement_Destination__c = 'Use/Create a New Disbursement Destination';
        Css[5].Existing_or_New_Disbursement_Destination__c = 'Use/Create a New Disbursement Destination';
        update Css;
        system.debug('Updated Css');
        test.stoptest();
        
        List<Case> insertedCases = [
            SELECT Type, RecordTypeId
            FROM Case
            WHERE Id IN :Css];
        
        system.debug(insertedCases.size() + ' Cases Have Been Created');
        system.debug(insertedCases);
        System.AssertEquals(insertedCases.size(),6);
        
        for(Case Cs : insertedCases ){
            System.AssertEquals('Disbursement Destination Already Populated',Cs.Existing_or_New_Disbursement_Destination__c);
            System.AssertNotEquals('',Cs.Disbursement_Destination__c);
        }
        
        List<Disbursement_Destination__c> TestDDs = [
            SELECT name
            FROM Disbursement_Destination__c];
        system.debug(TestDDs);
        //System.Assert(TestDDs.size(),2);
    }
}

Thanks again!