Hey Patrick,
The Required setting will actually be setup inside of your MailChimp account, rather than from the WordPress side of things.
To ensure whether you have any fields set to required on your list, login to your MailChimp account, then go to the Lists tab > Navigate to the list in question > click the gear/cog icon > Forms. From there, any required field should be indicated by a red asterisk.
Once you’ve ensured that the email address field is the only required field, log in back to your WP admin area, go to your MailChimp plugin settings, then click the ‘update list’ button.
If you’re still unable to submit the form as expected, we’d recommend going to your plugin settings, then clicking the “logout” button at the top. Then, log back in to your account, reconnect your list, and try signing up again.
We look forward to hearing back!
-tk