Appointments+ Set Up with Google Calendar

I believe I've set up Appointments+ correctly to use Google Calendar. However, this is the error I am encountering:

Warning: Cannot modify header information - headers already sent by (output started at /home/janste16/butterfly-maiden.com/wp-admin/includes/template.php:1642) in /home/janste16/butterfly-maiden.com/wp-includes/pluggable.php on line 876