Thanks for the reply. I did it like his but it does not work. It gives error Method does not exist.

public static testmethod void test3(){
    Account a = new Account(
            name = 'acc',
            Salesman_number__c = '1',
            Debtor_number__c = '123',
            UniqueDebtor__c = '1234',
            Sister_Company__c = false
        );
        insert a;
        
        // create order
        Order__c order = new Order__c(
            Account_Name__c = a.Id,
            Reserve_Allocation__c = false,
            Status_for_EWS__c='MOQ Check'
        );
        insert order;
        
         // create primary crop
        Primary_Crop__c pc1 = new Primary_Crop__c(
            BVC__c = 4,
            Current_Inventory__c = 300,
            Reserved_Stock__c = 20            
        );
        insert pc1;
        
        // create products with SKUs
        Product__c p1=new Product__c();
        p1.Primary_Crop__c = pc1.id;
        p1.name='KANGKONG';
        p1.active__c=true;
        p1.bvc__c=4;
        p1.Crop_Code__c='KANGKONG';
        p1.Product_Abbreviated__c='Kangkong 00004';
        p1.Product_Full_Name__c='Kangkong 00004';
        p1.Product_Name__c='Kangkong';
        p1.Commercial_Name__c='Kangkong';
        p1.Product_Number__c=1;
        
        insert p1;

     SKU__c s1=new SKU__c();
        s1.name='KANGKONG 010 IT PA2 20G IT';
        s1.Company_Code__c='EWIT';
        s1.Fill_weight__c=20;
        s1.Fill_Weight_Unit_Factor__c=0.001;
        s1.Fill_weight_unit_code__c='GR';
        s1.Packaging_Abbrev__c='PA2 4G IT';
        s1.pack_type__c='POUCH';
        s1.Price_Unit_Code__c='KG';
        s1.Price_Unit_Factor__c=1;
        s1.crop__c=p1.id;
        s1.Packaging_Number__c='134';
        s1.UniqueId__c = 'unq1';
        
        insert s1;
        
        // create order line items
        list<OrderLineItem__c> listOLI = new list<OrderLineItem__c>();
        OrderLineItem__c oli1 = new OrderLineItem__c();
            oli1.Order__c = order.Id;
            oli1.Crop__c = p1.Id;
            oli1.SKU__c = s1.Id;
            oli1.Unit_Price__c = 2.00;
            oli1.Quantity__c = 1;
            oli1.UniqueId__c = 'ext1';
            oli1.primaryCropId__c = pc1.id;
        
        listOLI.add(oli1);
        insert listOLI;
        OrderHandler OH = new OrderHandler();
        OH.updateUniqueIdinOrderLineItem(listOLI);
}