Error comes when i activate directory plugin

I get an error
when i activate directory plugin and navigation menu is set

"Warning: Cannot modify header information - headers already sent by (output started at /home/fresno9/public_html/californiadentistry.com/wp-content/themes/branfordmagazine-pro/header.php:2) in /home/fresno9/public_html/californiadentistry.com/wp-includes/pluggable.php on line 876"