[Support System] Support System Plugin not working

I installed support system on both http://www.caintos.com and http://www.onlinereading.club and tried to post a request. I got this message on the screen and no e-mail was sent to me. Any suggestions? Thanks!
Warning: Cannot modify header information - headers already sent by (output started at /home/readonline/public_html/wp-content/plugins/sharebox/lib/forms/box_template.php:15) in /home/readonline/public_html/wp-includes/pluggable.php on line 1216