Warning message on Contact Us page

Hi,

There is a warning message which has appeared across the top of our Contact Us (https://www.6-group.com/contact-6/) page which reads:

Warning: Missing argument 1 for plugin_dir_url(), called in /homepages/11/d678691150/htdocs/httpdocs/wp-content/themes/6group/functions.php on line 58 and defined in /homepages/11/d678691150/htdocs/httpdocs/wp-includes/plugin.php on line 723

Does anyone know what it is or how we can fix it?

Many thanks in advance,

Dan