We've only seen this message reported a handful of times in the past.
Copy and paste the contents of the document into a new document and the problem should go away. Apparently if something is corrupted in the file, copying and pasting it into the new document corrects the problem
The problem may be caused by having your documents mirrored on a cloud drive such as OneDrive or Dropbox. Screenwriter doesn't work well with cloud drives. Here are two FAQs to help you turn off the mirroring for OneDrive and Dropbox.
Restart your computer and reinstall Screenwriter can correct the problem if solution one doesn't work.
LINK: Download and Install Movie Magic Screenwriter 6 (2022) for Windows - WMM6
If neither of those solutions work, please send a tech support ticket through this website. If you can send us the document, we'll see if we can correct it for you and produce a full PDF of your document.