Thank you!
I just missed to write
if ( ! function_exists( 'is_plugin_active' ) ) {
include_once( ABSPATH . 'wp-admin/includes/plugin.php' );
}
while i was calling:
if ( is_plugin_active('advanced-custom-fields-pro/acf.php') ) {
/* My code goes here */
}
in my previous attempts.
Solved, Thank you very much!