The reason is that there is a limit to the number of audiences a Mailchimp account can have (the limit is different according to each plan) and one is already created when you first signup to Mailchimp.
Since FreeAgent CRM needs to be able to create an audience for the integration, you will need to delete an audience before attempting the pairing process. Alternatively, you can upgrade your Mailchimp account to have more audiences available. You can read more about the Mailchimp policy changes here.
Please also note that the integration with a Mailchimp free account is not possible due to other limitations. To learn more about these limits, please visit Mailchimp's pricing page.
Delete MailChimp Audience
Log in to your Mailchimp account and then click on 'Audience' (1) on the top menu. Then click the 'Manage Audience' dropdown menu to display the available options and click 'View audiences' (3).
This will take you to the current list of Audiences. You will notice a message (1) stating that your audience limit has been reached. Review Mailchimp's available option to determine if upgrading is the right options for you.
To delete the existing audience, click right next to its name.
Once you do, the 'Delete' button will appear on the screen. Click on it to proceed.
Since this process can't be undone, you will need to type 'DELETE' in the box to confirm.
Once you have deleted the Audience go to FreeAgent CRM and follow the steps described on the tutorial below to Pair your Mailchimp account.