Cannot Create Newsletter, receive error message

When I attempt to Create a Newsletter, I get the following message:

The website encountered an error while retrieving http://www.affinitymc.com/wp-admin/customize.php?wp_customize=on&theme=iletter&newsletter_id=2&return=admin.php%3Fpage%3Dnewsletters. It may be down for maintenance or configured incorrectly.

Any idea what's causing this?
Thanks!