Have you build the whole document and not shared any part of it?
Have some content shared with other users via Assigments, InCopy,.... Well if it sounds strange to you do not worry at all.
What I need some confirmation is if you have created the document, nor received or downloaded from somewhere else.
Do you know or have checked linked files? Placed pictures,... do they include any special character?
I just had this error message. For me, it turned out that my filepath was too long. My packaged folder was about 100 characters long. When I shortened it, it packaged without a problem. I didn't change any other settings.
Thanks for sharing it here. I am sure it'll help other users with similar concern.
Feel free to reach out to Adobe Community if you have any other issue. We'll be happy to help.