#1. Determine your cache location in pagespeed.conf (your location may vary from this)
sudo grep -i "filecachepath" /etc/apache2/mods-enabled/pagespeed.conf
# The ModPagespeedFileCachePath directory must exist and be writable
ModPagespeedFileCachePath "/var/cache/mod_pagespeed/"
# 2. Touch the cache.flush file
sudo touch /var/cache/mod_pagespeed/cache.flush
# 3. Restart apache
sudo service apache2 restart