If you are in the header then you are outside of “The Loop”. When you are outside of the loop you must provide ACF with the post ID of the post you want to get the value from because ACF cannot determine the correct post outside of the loop.
If what you are looking for is on a single post object then you would use something like
$queried_object = get_queried_object();
$group = get_field('caracteristicas', $queried_object);
