Checkout page broken

I cant seem to get the check/plans page to show up. I get this error:

Page Referenced: http://www.pictalo.com/?page_id=827

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /home/pictaloc/public_html/Pictalo.com/wp-content/themes/wapp/header.php:2) in /home/pictaloc/public_html/Pictalo.com/wp-content/plugins/pro-sites/pro-sites.php on line 632

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/pictaloc/public_html/Pictalo.com/wp-content/themes/wapp/header.php:2) in /home/pictaloc/public_html/Pictalo.com/wp-content/plugins/pro-sites/pro-sites.php on line 632
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