Fat core plugin have problem

Hello , I hope you be good.

I just purchased your theme and installed in my website, But your Fat core plugin is not working.
when i activated the plugin it is saying ( The plugin generated 4 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin. )

But i managed to install demo, But i can not Add/update/edit any page to the site. It is showing a blank page. Please help me to fix the problem as soon as possible.

Thank you