Welcome to the installer
We have detected that you don't have access to write to the system/cache directory. Under linux you can fix it by using this two command, where first one should be enough (for apache):
chown -R www-data.www-data /var/www/*
chmod -R 760 system/cache
chown -R www-data.www-data /var/www/*
chmod -R 760 system/cache