If you have Standard or Premium plans and you are still unable to pair then you will need to review your plan's limits. If you have reached the number of audiences available, you will need to remove an existing one or upgrade your plan to increase the limit.
This is necessary because FreeAgent CRM creates a new audience when you pair your email for the integration to work. You can read more about the Mailchimp policy changes here.
Delete MailChimp Audience
Log in to your Mailchimp account and then click on 'Audience' on the top menu. Then click the 'Manage Audience' dropdown menu to display the available options and click 'View audiences'.
This will take you to the current list of Audiences. If your have exceeded your limit, here you will see a message that will allow you to upgrade your current plan. Review Mailchimp's available option to determine if upgrading is the right option for you.
To delete an 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.