Registration error

Warning: Cannot modify header
I am getting the below error after I enter my username and password to register and then pay via paypal express

information - headers already sent by (output started at /home/jsgljc/public_html/alphaquantfx.com/wp-content/themes/SimplePress/header.php:2) in /home/jsgljc/public_html/alphaquantfx.com/wp-includes/pluggable.php on line 680