Newly installed – Error message

I’ve just installed the Mailchimp plug-in and have added the widget to the side bar. I’m using the Flymag theme.
I simply get the email box and “sign up”

When I enter the email it gives me an error message “Oops. Something went wrong. Please try again later.” I went to Mailchimp to check and seem to have done all that was asked. I’m no expert, and can’t code.

Have I missed something obvious?

Thanks….

https://wordpress.org/plugins/mailchimp-for-wp/