If you are actually entering json code into the field then json_encode()
is not necessary.
When getting fields outside of “The Loop” you must supply the post ID to get the field from
function acf_header_script() {
$queried_object = get_queried_object();
if (is_a($queried_object, 'WP_Post')) {
$schema = get_field('schema_code', $queried_object->ID);
echo '<script type=application/ld+json>' . $schema_code . '</script>';
}
}
add_action ( 'wp_head', 'acf_header_script' );