Getting a couple of errors on my checkout page that will not let me move on to the next step

errors i get are:

Warning: session_start() [function.session-start]: open(/home/content/08/12159008/tmp/sess_siaklv3snk3amqid7u7ouv2hj4, O_RDWR) failed: No such file or directory (2) in /home/content/08/12159008/html/wp-content/plugins/marketpress/marketpress.php on line 965

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/content/08/12159008/html/wp-content/plugins/marketpress/marketpress.php:965) in /home/content/08/12159008/html/wp-content/plugins/marketpress/marketpress.php on line 965

Due to this i have not been able to check out the process of making a sell on my website if this does not work what was the point in buying your software.