To enable ‘Newsletter Subscription’ and run it successfully, configure ‘Mailchimp’ keys under Admin->Settings->General settings->Third party API.

To collect these keys, Follow below steps:

1. Login to your Mailchimp Account, If you don't have one, create a new Mailchimp Account and login.(Visit https://mailchimp.com/)
   
   
2. Once logged in, you’ll be redirected to Mailchimp Dashboard. Click on ‘Username’ provided at the top-right corner, which will open a drop-down list of options.
   
   
   
   
3. Click on ‘Account’ which will redirect to the ‘Account’ settings page.
   
   
   
4. Click on ‘Extras’ which will open a drop-down list. Click on ‘API Keys’.
   
   
   
5. This will open the ‘API keys’ page.
   
   
   
6. Click on the ‘Create A Key’ button which will create a new ‘Mailchimp Key’. Copy and paste this API key in the corresponding section.
   
   
7. Click on ‘Audience’ provided on the top header of the page and then click on the ‘All Contacts’ button, which will redirect to the ‘Audience’ page.
   
   
   
8. Click on ‘Audience Name and Defaults'.
    
   
   
9. You’ll be redirected  to the ‘Audience Name and Campaign Defaults’ page. On this page, an ‘Audience ID’ is mentioned. Copy this ID and paste it in the ‘Mailchimp List ID’ input field.