I have to send excel report everyday for rejection order data. I am using Action flows to do so, but when the file is empty it suppose to send an email with no excel file as an attachment.

Your email is called sent email with CSV, why not make two email flows?

One with the CSV and one without, and then take one of those email flows based on the router?

Everything seems fine in the images. Can you show your below ( without attachment ) mail module's inside please ?

Something similar happened to me. Not sure if it will be the same, but just in case. It turns out that the issue on my case was that since I modified couple of times the action flow, I had left the source module celonis instead of the router. Maybe since for you is sending the attachment it has source module csv instead of the router.