The script checks whether fulfilment center Fulfilment Center Name has sufficient stock for all order items, if yes then the order is automatically allocated to the fulfilment center
Complete code (intialize and static method, copy bits inside Initialize method and IsOrderFolderExists static function)

namespace linnworks.finaware.CommonData.Objects                // leave untouched                            
{                                                                                               // leave untouched                            
    public class ScriptMacroClass : linnworks.scripting.core.IOrderScript                       // leave untouched                            
        {                                                                                       // leave untouched                            
            public void Initialize(linnworks.finaware.CommonData.Objects.Order order,linnworks.scripting.core.Debugger debug)           // leave untouched                            
            {                                                                                   // leave untouched                            
            
            /*
            * This script will get all stock items
            */
            
                        
            System.Collections.Generic.List<System.Guid> list = new  System.Collections.Generic.List<System.Guid>();        // declare a list of unique ids
                
            bool SufficientStock = true;// bool to flag if we have sufficient stock
            string FulfilmentCenterName = "Fulfilment Center Name";                                // the name of the fulfilment center
            
            
            // lets collect all order items in to a list, so that we can bulk check the stock level for a specific location
            foreach(OrderItem i in order.OrderItems){                                            // itterate through all order items
                if (i.pkStockItemId != System.Guid.Empty){                                        // is order item linked?
                    list.Add(i.pkStockItemId);                                                    // if yes add to the list
                }
                else if(!i.IsService && i.pkStockItemId == System.Guid.Empty) {                    // if not linked 
                    debug.AddEntry(i.ItemNumber + " is unlinked");                                // add line to debug 
                    SufficientStock = false;                                                    // no sufficient stock - we have an unlinked item in the order
                }
            }        
            
            // we can get stock levels for a list of items in bulk using ListStockAvailablityInLocation method
            System.Collections.Generic.List<linnworks.finaware.CommonData.Inventory.StockItemAvailability> availability = linnworks.finaware.CommonData.Inventory.ListStockAvailablityInLocation(list, FulfilmentCenterName, order.GetConnectionString);
            
            // iterate through the result set of ListStockAvailablityInLocation
            foreach(linnworks.finaware.CommonData.Inventory.StockItemAvailability aItem in availability){
                debug.AddEntry(aItem.SKU + "=" + aItem.Available.ToString());            
                if (aItem.Available <= 0){                                                        // if we have insufficent stock for one of the items
                    SufficientStock = false;                                                    // set no Sufficient stock
                }
            }
                
            if (SufficientStock){                                                                //if we have sufficient stock 
                order.SetFulfilmentCenter(FulfilmentCenterName);                                // allocate order to a fulfilment center
                order.SetUnassignFromFolder("Waiting Fulfilment Center Stock");                    // remove order from folder "Waiting Fulfilment Center Stock" just in case it was assigned to a folder before
                debug.AddEntry("Order allocated to " + FulfilmentCenterName);                    // output something in debug
                order.Save(0);                                                                    // SAVE ORDER
            }
            else{                                                                                // if no sufficient stock
                
                if (!IsOrderFolderExists(order.OrderFolder, "Waiting Fulfilment Center Stock")){//check if the order is allocated to a folder "Waiting Fulfilment Center Stock"
                    OrderFolder folder = new OrderFolder("Waiting Fulfilment Center Stock");    // if not, create new folder object
                    order.OrderFolder.Add(folder);                                                // assign to an order
                    debug.AddEntry("Allocated to folder");                                        // debug line
                    order.Save(0);                                                                // SAVE ORDER
                }
                                
                debug.DoNotLogScriptRun = true;                                                    // here we need to tell Linnworks not to treat this order as SCRIPT_RUN and re-execute this script next time we sync for this order
            }
        
            
            }                                                                                   // leave untouched                            
        
            
            /// <summary>
            /// static method to check if an order is assigned to a folder, 
            /// here we simply itterate through all folders and check if folderName = FolderName
            /// </summary>
            static bool IsOrderFolderExists(System.Collections.Generic.List<OrderFolder> list,string folderName){
                foreach(OrderFolder f in list)
                    if(f.FolderName == folderName){return true;}
            
                return false;
            }
    }                                                                                           // leave untouched                            
}                                                                                               // leave untouched