$user_id = get_the_author_meta( 'ID' ); 
$user_obj = get_userdata( $user_id );
if( !empty( $user_obj->roles ) ){
    foreach( $user_obj->roles as $role ){
        echo $role;
    }
}
?>
Single.php de gözükmesini istediğin yere koy.
$user_id = get_the_author_meta( 'ID' ); 
$user_obj = get_userdata( $user_id );
if( !empty( $user_obj->roles ) ){
    foreach( $user_obj->roles as $role ){
        echo $role;
    }
}
?>