The pickup date cannot be copied into existing schedule lines

What causes this issue?

You have changed the pickup date.
When you create a delivery order the system uses the pickup date as the
default delivery date for the individual schedule lines. If the pickup
date is changed later, the <DS:GLOS.delivery_date>delivery date</> in
the schedule lines is not adjusted. This means that any manual changes
that might have been made in the meanwhile are not overwritten.

System Response

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

How to fix this error?

Use the fast change function to adjust the delivery date in the
schedule lines that haven't been delivered so that it matches the
changed pickup date.

Error message extract from SAP system. Copyright SAP SE.