This error you've reported is a Wordpress shortcode and is no longer supported in 3CX Live Chat. It was a shortcode that existed in a previous version of Live Chat which was unlisted from the Wordpress Plugins Marketplace.
Ensure that you've removed the previous versions of 3CX Live Chat entirely and manually remove the shortcode you've presented, and then download the latest supported version of the plugin.
The shortcode may exist in your Footers or Pages. Check your pages in HTML code view to see if it exists, if not check Appearance >> Widgets >> Footer and ensure the short code is not there either.
The website was previously built using a plugin called Elementor which doesn't allow access to the html code.
Do you think if I install the newer version of the plugin, it will over-write the previous code? I do see the code in inspect element via Chrome. Check it out for yourself: systemforce.co.uk
You can try but I don't think it will overwrite it.
You'll need access to the HTML code. Which means you'd need to switch to the classic editor rather than the Elementor editor. Remember to take backups of the site before making any changes.
There are many possibilities that could have caused this. Things like:
1) Cached pages in an html cache system before the deactivation of elementor: solution -> purge site cache
2) Elementor is still enabled. This shortcode is inside an elementor-widget: solution -> check and remove that elementor widget
3) Many other conflicts in his site when the elementor was uninstalled