This error message is generated when you attempt to format a Word document in Write-N-Cite that has a file extension of .docx. At this time, Write-N-Cite is not able to process .docx documents. Save your document as a .doc file, and try running your bibliography again.
If you require the functionality of a .docx document, try using RefWorks to process your bibliography, rather than Write-N-Cite.
Click on the "Bibliography" tab and select "Create" from the dropdown menu.
In the pop-up window that comes up, select the bibliography style you'd like, and browse for the paper you've saved to your computer.