'''Message failed to send to reservation 12345678 - content or attachment is required'''
There are two possible reasons:
1. There is no content in the language of the Guest(OR the default language you have set for your account, ) in the Messaging tab of the auto action that you triggered for this booking.
*Open the booking go to the Mail & Action tab and check the guest language.
*Open the auto action and ensure you have text in the Subject, Plain Text Field and HTML Field.