Hello,
Are you able to do the transformation using PHP CLI mode?
Please check that LibreOffice is closed and try the first and second steps explained on:
https://www.phpdocx.com/documentation/conversion-plugin/debugging-libreoffice
First test is running directly the main exec of LibreOffice and then with PHP CLI mode. Is the PDF generated?
Also please check the common issues page and the info about using it on Windows:
https://www.phpdocx.com/documentation/conversion-plugin/common-problems-and-possible-errors
Regards.