This error comes up when an item is attached as detail to a specific task the tech has added to the invoice, and he/she does not add a serial number to the part in the Parts Used section.
Locate which task was used that has detail attached to it from SuccessWare21.
On the Payment screen, tap the Parts Used button
Tap the task associated with the part that needs to have a serial number on it
Locate the part in the list of parts, and enter a serial number for that specific part, and click Save.