The config file /var/www/matomo/config/config.ini.php seems to exist but Matomo could not read it.
» Please check that /var/www/matomo/config/config.ini.php is readable by the user 'www-data'.

Could not find the Matomo configuration file, and you are trying to access a Matomo page.
» You may install Matomo now
If you installed Matomo before and have some tables in your database, don't worry, you can reuse the same tables and keep your existing data.