When I activate de Appointments+, appear this message

When I activate de Appointments+, appear this message on the footer:

Warning: Unknown: open(/var/lib/php-cgi/session/sess_rad16o6q780l2dgrt5jlladh80, O_RDWR) failed: No such file or directory (2) in Unknown on line 0

Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/lib/php-cgi/session) in Unknown on line 0

Anybody help me?