Hello:
I had the same error message on Wnindows7 with Libreoffice.
I had incorrect configuration in my config/phpdocxconfig.ini
It is important to have the "single quote" for LibreOffice executable path.
See :https://www.phpdocx.com/documentation/conversion-plugin/conversion-plugin-installation-and-set-up
[transform]
; libreoffice or openoffice
method = "libreoffice"
; libreoffice installation path, absolute path -- wrap with single quotes
path = '"C:\Program Files (x86)\LibreOffice 5\program\soffice.exe"'