I enabled WP_DEBUG
It said that this line
add_action(‘cred_save_data’, ’my_save_data_action’,10,2);
should be this (" instead of ')
add_action("cred_save_data", "my_save_data_action",10,2);
when I made those changes, it returned no more errors, but it still does not work. The author field remains empty after publishing by a guest.