Appointments+ Google Calendar sync not working.

This is the error I get when I test my google calendar connection.

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

I've read previous documentation on this and am still having the same problem.