I received this by email not long ago.
Can you try adding the API key below, when I do it I get the error message. I’ve never had this problem before and I am hoping you as owner may have workaround? API Hidden

First up, this is a Mailchimp error, not Ourlocality, as the roles (Owner, Admin, or Manager) are not related to WordPress, but Mailchimp.
It should not make any difference who owns the website itself, as even the “super admin” got the same error with the same API, which BTW is not something that one should share widely.
A different API worked fine, so there is something wrong with the Mailchimp API.
Our Answer
- Check the API page, which should list all attempts to use it, it may have a more helpful error
- Make sure API key is being used for for this one application only
- Test a different API key, by creating a new one
This is a Mailchimp plugin, so they should be able to provide support to paying users for errors generated at their end.
Check this post on how to raise a ticket, if you pay for a mailchimp account:
https://wordpress.org/support/topic/api-key-must-belong-to-owner-admin-or-manager/
and the general instructions:
https://kb.mailchimp.com/lists/signup-forms/connect-or-disconnect-list-subscribe-for-wordpress