Since WordPress 5.2 there is a built-in feature that detects when a plugin or theme causes a fatal error on your site, and notifies you with this automated email.
In this case, WordPress caught an error with your theme, Paeon.
First, visit your website (http://www.charllaas.com/) and check for any visible issues. Next, visit the page where the error was caught (http://www.charllaas.com/wp-admin/admin.php?page=nicethemes) and check for any visible issues.
Hello,
Thank you for reaching out. I hope you’re doing well.
Are you on the latest version of Paeon? The latest version with the latest WP is not showing any fatal.
Best,
Juan.