Piwik couldn't write to some directories.
Try to Execute the following commands on your server:
chmod 0777 /var/www/stats/tmp/templates_c
chmod 0777 /var/www/stats/tmp/cache
chmod 0777 /var/www/stats/tmp/assets
If this doesn't work, you can try to create the directories with your FTP software, and set the CHMOD to 0777 (with your FTP software, right click on the directories, permissions).
After applying the modifications, you can refresh the page.
If you need more help, try Piwik.org.