session_start error

Hi,

I get this error on my page

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home1/onigetoc/public_html/reveilletoi.com/wp-content/themes/fashionistas/header.php:11) in /home1/onigetoc/public_html/reveilletoi.com/wp-content/plugins/tt-guest-post-submit/tt-guest-post-submit.php on line 30

https://www.ads-software.com/plugins/tt-guest-post-submit/