Can’t Update function.php after writing short code

The answer by @motivast is correct. However, to avoid using a billion echo and apostrophes, you can simply close the PHP tag and open it after your script:

function wpse_287665_mobile_menu_script() { ?>

    <script>
        jQuery(document).ready(function($){
            $("button.menu-toggle").click(function(){
                if( $("button.menu-toggle").attr("aria-expanded") == "true" ) {
                    $("button.menu-toggle").attr("aria-expanded", "false");
                    $("ul#top-menu").attr("style", "display:none;");
                } else {
                    $("button.menu-toggle").attr("aria-expanded", "true");
                    $("ul#top-menu").attr("style","display:block;");
                }
            });
        });
    </script><?php
}

add_action( 'wp_footer', 'wpse_287665_mobile_menu_script' );

Neat, isn’t it?

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