Piwik couldn't write to some directories (running as user 'www-data').
Try to Execute the following commands on your server, to allow Write access on these directories:
chown -R www-data:www-data /var/www/piwik
chmod -R 0755 /var/www/piwik/tmp
chmod -R 0755 /var/www/piwik/tmp/assets
chmod -R 0755 /var/www/piwik/tmp/cache
chmod -R 0755 /var/www/piwik/tmp/logs
chmod -R 0755 /var/www/piwik/tmp/tcpdf
chmod -R 0755 /var/www/piwik/tmp/templates_c
If this doesn't work, you can try to create the directories with your FTP software, and set the CHMOD to 0755 (or 0777 if 0755 is not enough). To do so with your FTP software, right click on the directories then click permissions.
After applying the modifications, you can refresh the page.
If you need more help, try Piwik.org.