Most of the time this problem occurs when your Sale Trade Condition (STC) is not properly filled out and applied correctly to the Sale Order (SO)!
Make sure that your STC has a valid Effective Date and Expiration Date in the future. Also make sure that Supplier / Seller and Buyer are correctly related in VIEW tab and PROFILE tab.
Furthermore make sure that you "Apply Sale Trade Condition" in the action Menue in your SO after this. When successful and you switch to the related Sale Packing List, you will find a red text above telling you that the Sale Packing List is related to your Sale Order.