Hello,
If you are using an Advanced or Premium license, you need to download the namespaces package from MY PHPDOCX page (on this page you can download a classic package and a namespaces package).
After extracting the content of the package you just need to create the folder vendor/phpdocx and follow the instructions detailed on https://www.phpdocx.com/documentation/cookbook/integrate-phpdocx-with-symfony2-symfony3
Regards.