Add footer.php to WordPress child theme

You have wrong argument for the filter. The first argument of the add_filter is the filter you are hooking up to. In your case it’s the mesmerize_get_footer_copyright. The second argument is the Call back function that would run the filter is called.
So change your add_filter to this

add_filter('mesmerize_get_footer_copyright', 'change_copyrightText', 10,1) ;