You will want to ensure that you have a functional list chosen for your MailChimp integration. You'll choose your list by navigating to Accumula.co > Apps > MailChimp. Here, you can choose which list from your MailChimp account you'd like to use.
To find your MailChimp lists, you'll want to navigate to MailChimp > Lists. To edit a list, find the list you want to edit and click the arrow on the right and choose Settings, then choose "List fields and *|MERGE|* tags". Your list should have a minimum of the following fields (the types and tag names are required!):
You're welcome to add more fields, but you must make sure that they are NOT required. In the end, it should look similar to the below: