Failed to write session data (files)

You need set permission for your var directory. That seems to be the problem.

Default ownership may be incorrect on session folder: Example; php-fpm7.0 on Amazon Linux defaults to apache user. If using nginx or other need to switch the folder ownership. Also as a note you have to change the user/group setting in www.conf.

chown -R root:nginx /var/lib/php/7.0/
sed -i 's/apache/nginx/g' /etc/php-fpm-7.0.d/www.conf
service php-fpm-7.0 restart