This issue was caused by not having a default stock type for a delivery charge, you can set this by doing the following:
Click Options, System Maintenance, System Settings
Choose GENERAL System Settings
Click Control File
Search for DELIVERY_STOCK_TYPE
Enter in the code for the stock type you use for delivery charge. Now when you try and raise Purchase Orders through MRP or auto generate purchase orders it will allow you to add a Delivery Charge.
