Hi,
You can create the shipment cost document in VI01 transaction, with reference to the shipment document. The freight values can be pulled based on the condition records maintained (in TK11) or it can be inserted manually in the shipment document, based on the pricing procedure assigned.
You can check the errors using the error log and you can go to the item details - and go to conditions tab where you can check the conditions triggered and the manual conditions, if required. Refer the screenshot from VI01:
Regards,
AKPT