Defult Post thumbnail

[orisin]

Hi
I've been trying for a while to use the defult post thumbnail plugin at WP unsuccesfully
I'm using the Authority pro theme and plugin and I was following the instructions on the plugin page (add add_theme_support( 'post-thumbnails' ) to functions.php) but it just doesn't work
in the setting page I get the following message:

Warning: Cannot modify header information - headers already sent by (output started at /home/learnhe1/public_html/mybellylosssecrets.com/wp-admin/admin-header.php:19) in /home/learnhe1/public_html/mybellylosssecrets.com/wp-content/plugins/authorityproengine/authorityproengine.php on line 2528
Your theme does not have post thumbnails support enabled

Anyone knows how to make it work by any chance?
Thank you