Hello,
It seems the DOCX doesn't contain a valid .rels file, or PHP is unable of read it.
Please run the included sample DigitalSignature\signDocx\sample_1.php using PHP CLI mode and reply the topic if it works or not (note that signdocx overwrites the DOCX to be signed to add the signature).
If the included sample works, please send to contact[at]phpdocx.com the DOCX you are trying to sign and the dev team we'll check it.
Which version of PHP are you using? The digital signature module is fully tested with PHP 5.2 to PHP 7.
Regards.