This issue occurs when the document is in an unsupported file format. Apsona Document Generator only supports .docx files (Word 2007 or later), and does not recognize legacy .doc files. As a result, documents saved in .doc format will not appear at all in the merge action dropdown.
To resolve this, convert the document to .docx format by opening it in Word and saving it as a newer file type. Then upload the converted file to the Salesforce Documents object (not Files), ensure it is stored in a public folder, and clear the Apsona cache (Settings → Clear Cache) to refresh availability.
Once these steps are completed, the document will appear in the merge dropdown. Always ensure documents are in .docx format, as unsupported formats like .doc will not be detected by Apsona.