This issue typically occurs when the source report used for the merge does not include the record ID field in this case, the Opportunity ID. Step 4 of the merge setup needs the record ID to identify and update the correct Opportunity records.
To resolve this:
Add the Opportunity ID field to the columns in the single-step report.
Reopen and re-save the merge action to refresh the setup.
If the field still doesn’t appear, clear your Apsona cache via Settings > Clear Cache and check both boxes.
Once completed, the Acknowledgment Status field should appear in Step 4, allowing you to configure the field update successfully.