Same issue here. I double checked the mailchimp authentification and the form. I also used – to be sure – the butcher demo website newsletter-form. Form does contain an email field, mailchimp connection successful, everything setup properly according the tutorial. Write to wordpress database works, my entries in wordpress are saved, but not to mailchimp.
Seems there is a bug. Please, Avada-Team, since here are a bunch of users struggeling, could you please investigate if there is a bug in the integration when it comes to write data to mailchimp?