How to fix: "shell_exec() has been disabled for security reasons"

Yesterday I opened my website and on the top of the page there is this: "shell_exec() has been disabled for security reasons in /web/htdocs/https://ift.tt/2JdSGik on line 178"

It's written twice and I noticed also that all the sliders in the website aren't in the right size.

I searched for this code and on lines 177-178 appears this: if ( ! $port_file_content ) { $response = shell_exec( 'test -f /etc/yum.repos.d/baseos.repo && echo Avalon' );

How can I fix it? Thank you.