Reading the error verbatim, your webserver does not have access to the sessions folder in your filesystem. Apache may be running as another user (like apache or www-data) that can't edit your session dir.
You need to change your permissions on /var/lib/php/session/. Set the permissions of your existing session dir to be
chmod u=rwx,og=wxt /var/lib/php/session/
If that doesn't work or can't be done, change the directory PHP stores sessions to something safe like /tmp
Check your php.ini settings or use this:
So have a look here and feel better that others are suffering too: