Hello,
It's not returning a PHP error but a PHP notice that you can hide if needed (https://www.php.net/manual/es/function.error-reporting.php), or directly using:
@$docx->embedHTML(file_get_contents($htmlFile));
If you don't want to hide PHP notices, please set a border width value for the table you are adding when importing HTML. We have sent this topic to the dev team to check the PHP notice.
Regards.