Hello,
Welcome to the Bacularis user group.
This error is a missing PHP session module. I mean that it can be installed by default with PHP but it can be commented in PHP config. In FreeBSD you need to make sure that in this file:
/usr/local/etc/php/ext-18-session.ini
the session extension is uncommented. It should be:
extension=session.so
but not:
;extension=session.so
After that you need to restart the web server (or php-fpm if you use it).
Please let us know if it helped.
Good luck.
Best regards,
Marcin Haba (gani)