Hello,
We think the warning appears because the ODT can't be generated. Maybe there's some missing rw access to the target or .wapi folder.
We recommend you to follow the debug steps available on https://www.phpdocx.com/documentation/conversion-plugin/debugging-openoffice, to know the exact problem.
We also recommend you to use the conversion plugin based on LibreOffice instead of the OpenOffice one. The conversion plugin based on LibreOffice is the recommended one since phpdocx 4.5 to get the best output; on https://www.phpdocx.com/documentation/conversion-plugin you can read all information about the conversion plugin and the steps to install and configure it.
Regards.