The error (Web Request! Error: An XML error! - Error 1: piece segment weight field value is invalid Error 2: piece segment weight field is invalid) occurs when the weight in the order details is incorrect. To resolve this issue, follow these steps:
Click Order, then Overview.
Find the Order.
Click Actions, then Details.
Scroll down to the Weight field and update it to match the weight shown in the order details.
Click Save.
Once the weight is corrected, the error should no longer appear.