Hi Inoruhana,
If you want, you can safely ignore that, the actual WCAG AA check is positive, button labels are not mandatory.
However if you want to add a button label, you can do that by editing the core plugin code.
– Make sure to have a full server backup first
– Open up the wp-content/plugins/ajax-search-pro/includes/views/asp.shortcode.probox.php file on your server
– Replace it’s content with this.
– Save, and the issue is solved.