This is usually caused by your php sessions not being set-up correctly.

In 99.99% of cases this is due to the session save path (php.ini) not being set correctly or at all and/or the directory not existing or not being writable.

Typically this would read
session.save_path = "/tmp"
or
session.save_path = "/var/lib/php5"
or
session.save_path = "/var/lib/php5/session"
or similar in your php.ini.

Refer to the php manual for more details regarding session setup

You should speak to your host / server administrator to get this fixed and to make sure the appropriate path is set with the correct permissions. WPPizza will not work without php session support.