Appointments + giving me two issues is the site

1st issue: When I click on an appointment it does nothing.

2nd issues: When I test the connection I get “Warning: Cannot modify header information – headers already sent by (output started at /home/fasbreak/public_html/wp-admin/includes/template.php:1642) in /home/fasbreak/public_html/wp-includes/pluggable.php on line 876”

I have gone over the install instructions three times now and realize I must be missing something simple.

I also figure the issues are related.

I really appreciate any help you all can give me. Thanks.