I get this error after entering all the details i want for

I get this error after entering all the details i want for the pop and then when i save it i get this

Warning: Cannot modify header information - headers already sent by (output started at /home/surferso/public_html/new/wp-content/plugins/incsub-support/inc/classes/shortcodes/class-abstract-shortcode.php:12) in /home/surferso/public_html/new/wp-includes/pluggable.php on line 1207