Error when accessing administration config of MailChimp module in Drupal:

Message ParseError: syntax error, unexpected 'Config' (T_STRING), expecting function (T_FUNCTION) or const (T_CONST) in Composer\Autoload\includeFile() (line 20 of /var/www/html/web/modules/contrib/mailchimp/src/ClientFactory.php)

There are many and various use cases of how you would allow your users to login into your website. Most will require a username, while some requirements will allow only the user name OR just the email address. With Drupal there are many ways to do any of these thru Drupal modules.