Hustle install issues

Trying to install and configure Hustle. When installed I am getting http 500 errors on my site when in test mode. I tried to register my MailChimp API for the provider and it returns the message Something went wrong and does not register. I am using the free service for MailChimp, does this affect API usage?

I can't share the API info on here for security.

Would be grateful for any help getting this going - currently trying to get established with some of the WPMUDEV plugins and remove LeadIn Free which we have been using for lead capture.