The system uses the incomplete procedure to determine which field appears in the incomplete log when the user does not enter information during sales order processing. The error group is assigned to your partner function which controls further processing when a required entry is missing.
So if your partner function is missing in the sales document then you'll get the error and it depends on the customization for incomplete procedure that you assigned to your partner function in the error group 08 (Partner type vendor).
Solution: Make the necessary entries in your sales document which you defined in the error procedure 08.