The message may suggest that some form of document protection been applied to the document.

You can upload an example document which illustrates the issue. Make a copy of the original document, remove anything sensitive and then use the Manage Attachments button at the bottom of the thread. If you can't see that option, first click the Go Advanced button at the bottom of the thread.

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional