Business Scenario:
During the Confirmation of the Process Order, which is activated with Backflush (Auto GI, during the confirmation), System is not allowing the Additional Good issues/increase the Component Quantity manually. Getting the Error message, when saving the Confirmation.

Execution Steps as follws below
Create the Process Order using the T Code: COR1
In this Process Order, Components are activated with Backflush Indicator, which will enable automatic goods issue during the confirmation. The Component Quantities are 5 L for each of the component

Release the Process Order using theT Code: COR2

This order is in the Released & saved in the change mode.
Confirm the Process Order using the T Code: COR6N

During the confirmation, User is trying to add the additional goods issue (or) increasing the Component Quantity manually.
In the Goods movement tab>>Increasing the component Qty from 5 to 10 (more than 5)

After manually Increasing Component Quantity, then Click on Save >> Getting the below Screen

If we look at the Error details
System tis throwing the Error, for this additional Goods Issue, changed manually

Root cause of this Error
Look at the System Message control stetting in the Configuration using  T Code: OMCQ

Solution Proposed:
If we change Error message to Warning message, (E to W) System will allow the Additional (or) Excess Goods issue of the Component Quantities manually, during the Confirmation.

Hope, this is of some Useful Info. Kindly update with your  Valuable Feedback/Comments/Inputs.
Thanks & Regards
Raja Sekhara Reddy Bannuru