solution
chmod 777 -R /opt/lampp

from venezuela

Hello,

please confirm you changed the permissions of your var folder within the website’s directory? It’s a common misunderstanding and you might’ve changed the permissions of the server’s /var/ folder.

One more thing, I would recommend changing the permissions recursively of the var/cache folder.

Solution:

sudo cd /var/www/prestashop
sudo chown -R www-data *
sudo chgrp -R www-data *