MailChimp + Drupal 8: How to install properly? (page could not be found)
You should use Composer to fetch this module, as it has an external dependency noted in its composer.json file:

http://cgit.drupalcode.org/mailchimp/tree/composer.json

It seems to include an older version with the module, I'm not sure why, but the current version of Mailchimp API by Thinkshout looks to be 1.0.8. I don't see any installation notes. The module comes with 1.0.6... ?

Other than that, clear your caches and try to access the page again.