Hi @tarou chabi ,
Regarding "add an empty attachment to a SPO list column", when creating a new item, add attachments. Next, make the attachments column visible. By default, the Attachments column is hidden. To unhide it, you can click on one of the column names and select column settings > Show/hide columns, and then tick the Attachments column.
However, this column only indicates with an attachment icon whether the items has attachments or not. It does not show the detailed attachment. If someone wants to open the attachment, he/she still needs to open the list item. By default, attachments are opened directly in the browser.
Regarding "I want to create an approval workflow.", can you describe your requirements in detail?
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If this is helpful please accept answer.