Adjusting JPEG compression via functions.php file (works only in theme file!)

have you tried this in your child-theme functions.php:

function av_return_100(){ return 65; }
add_filter('jpeg_quality', 'av_return_100');
add_filter('wp_editor_set_quality', 'av_return_100'); 

i have no errors on that before:
Enfold is managing this in functions-enfold.php line: 2318
(newest Enfold 4.7.2 )

if( ! function_exists( 'av_return_100' ) )
{
	/**
	* Sets the default image to 100% quality for more beautiful images when used in conjunction with img optimization plugins
	*
	* @since 4.3
	* @added_by Kriesi
	*/
	function av_return_100(){ return 100; }
	add_filter('jpeg_quality', 'av_return_100');
	add_filter('wp_editor_set_quality', 'av_return_100');
}

you allways can replace an existing parent function if it is introduced by if( ! function_exists(…
in the child-theme functions.php.
so i only used the existing function : av_return_100

there must be a different error. This is a fundamental behavior of WordPress itself.
If there is a “Pluggable-Function” used – and this is allways the case if it is built via :

if (!function_exists('my_parent_theme_function')) {
    function my_parent_theme_function() {
        // Code of your pluggable function
    }
}

you can allways override the function this way (without the if-clause) in child-theme functions.php with :
( because – child theme functions are being executed first in WordPress)

    function my_parent_theme_function() {
        // Code of your pluggable function
    }

so maybe you are willing to post your whole child-theme functions.php here – to see if there are additonal errors.

_________

a link f.e.: https://mhthemes.com/support/knb/overriding-parent-theme-functions-child-theme-tutorial/