This typically happens when the source report used for the merge does not include the required record ID field. For the condition field to appear in the setup, Apsona needs to know which specific records to reference, and this is possible only if the corresponding record ID (e.g., Opportunity ID for Opportunity objects) is explicitly present in the report.
To fix this:
Add the appropriate record ID field to the report columns.
Reopen and re-save the merge action to refresh the setup.
If the field still does not appear, clear your Apsona cache via Settings > Clear Cache by selecting both checkboxes.
Once these steps are completed, the field should be available for selection in the condition field list.