A family of Microsoft word processing software products for creating web, email, and print documents.
Are you using the "New Outlook"
It is just a re-badged version of Windows Mail and, like its predecessor, it cannot be used to send messages created by using the mail merge facility in Word,
If that is what you are using, you will need to switch to the original Outlook by using the toggle switch at the top right of the Outlook Window
If you do not see the above toggle, the toggle for Outlook Classic may be under Help: