Specify target entries.

What causes this issue?

You have selected an entry to be copied (as a reference entry).

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?

You can now edit the field contents. Please note that the key must be
changed because the system cannot store a second entry with the same
key.

Error message extract from SAP system. Copyright SAP SE.