How to resolve the error when to invoice an order,the system throws an error: “An unexpected error has occurred”

The system will generate an error when trying to bill an Item Fulfillment when the shipping method chosen on the shipping tab 

1. Navigate to Lists > Accounting > Shipping Items.
2. Click Edit next to the shipping item in question – in this case, the shipping method chosen on Item Fulfillment that you are trying to Bill.
3. Click Shipping Rate tab.
4. Select a GL account in Account (Shipping) field.
5. You may also have to select an option on CHARGE TAX ON THIS SHIPPING PORTION OF ITEM field on Shipping Rules tab since this is a mandatory field.
6. Click Save.