I keep getting the following message on my STORE

Hello,

I keep getting the following message on my STORE and PRODUCTS pages.

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/bikeworks/public_html/wp-content/themes/pro3/header.php:2) in /home/bikeworks/public_html/wp-content/plugins/marketpress/marketpress.php on line 993

Also, the {Confirm Payment} button isn't working.