Change of final delivery indicator for reservation & item & not possible.

There is a flag for the final delivery indicator with reference to a
reservation. Reservation can only be used once for one shipment.
Therefore the system automatically sets the final delivery indicator
when posting the transfer.
Error message.
Do not set or reset the final delivery indicator for reservations.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.