I am not sure how to resolve this.  With the new Orders Object Salesforce does not give you the option to create a new Order from the Opp.  This is a requirement for our org.  I have created a custom VF page and am using the Orders standard controller along with an extension to add some other functionality.  Everything is working, but I cannot get my test class to pass because the AccountId field is not writeable.

    public  class SaveAndAddProductControllerExtension {
    
    private ApexPages.StandardController stdCtrl;
    public Contact OrdContact {get;set;}
    public Contact OrdContact2 {get;set;}
    public Opportunity Name {get;set;}
    public Opportunity ActName {get;set;}
    public string id;
    public string Acid;
    Order ord;
    
    
        private ApexPages.StandardController controller;
        
            public SaveAndAddProductControllerExtension(ApexPages.StandardController controller) {  
                
                stdCtrl = controller;
                ord = (order)controller.getrecord();
                id = ApexPages.currentPage().getParameters().get('oppid'); 
                Acid = ApexPages.currentPage().getParameters().get('Accid');
                if (id != null)
                {
                name = [select id, name from opportunity where id= :id];
                }
                ord.Opportunity_Name__c = id;
                ord.AccountId = acid;
                this.controller = controller;
                }
    
        public void ContactPopulated(){
            Order cont=(Order)stdCtrl.getRecord();
            OrdContact =[select id, MailingStreet, MailingCity, MailingState, MailingPostalCode from contact where id= :cont.BillToContactid ]; 
            }
    
        public void ContactPopulated2(){
            Order cont=(Order)stdCtrl.getRecord();
            OrdContact2 =[select id, MailingStreet, MailingCity, MailingState, MailingPostalCode from contact where id= :cont.ShipToContactId]; 
            }
        
        public Order getOrder() {
            if(ord == null) ord = new Order();
                ord.EffectiveDate = System.Today();
                ord.EndDate = System.Today() + 1095;
                ord.Status = 'Open';
                return ord;
        }
    
        public PageReference save() {
            try {
            system.debug('*************' +Ord);
            ord.BillingCity = OrdContact.MailingCity;
            ord.BillingPostalCode = OrdContact.MailingPostalCode;
            ord.BillingState = OrdContact.MailingState;
            ord.BillingStreet = OrdContact.MailingStreet;
            
            ord.ShippingCity = OrdContact2.MailingCity;
            ord.ShippingPostalCode = OrdContact2.MailingPostalCode;
            ord.ShippingState = OrdContact2.MailingState;
            ord.ShippingStreet = OrdContact2.MailingStreet;
            insert Ord;
    
            PageReference neword = new PageReference('/apex/orderProductEntry?Id='+Ord.Id);
            system.debug('test::::::::::::' + neword);
            neword.setRedirect(true);
                return neword;
            }
            catch (DMLException e) {
                ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR, 'Error creating new order.'));
            return null;
        }
    
    
      }
    
    }