Hello,
The TransformDoc.inc file is an old file that was deprecated since the release of phpdocx 4; the official solution to transform files is using the conversion plugin. Also note that this class is not to open a DOCX but to transform it to HTML.
Although it's not longer supported, it's still included for very simple documents. Please check that you're setting the correct path to the TransformDoc.inc file, it seems that's the error.
Regards.